[PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[paulmck]

From: "Paul E. McKenney" <paulmck@kernel.org>

There are kernel facilities such as per-CPU reference counts that give
error messages in generic handlers or callbacks, whose messages are
unenlightening.  In the case of per-CPU reference-count underflow, this
is not a problem when creating a new use of this facility because in that
case the bug is almost certainly in the code implementing that new use.
However, trouble arises when deploying across many systems, which might
exercise corner cases that were not seen during development and testing.
Here, it would be really nice to get some kind of hint as to which of
several uses the underflow was caused by.

This commit therefore exposes a mem_dump_obj() function that takes
a pointer to memory (which must still be allocated if it has been
dynamically allocated) and prints available information on where that
memory came from.  This pointer can reference the middle of the block as
well as the beginning of the block, as needed by things like RCU callback
functions and timer handlers that might not know where the beginning of
the memory block is.  These functions and handlers can use mem_dump_obj()
to print out better hints as to where the problem might lie.

The information printed can depend on kernel configuration.  For example,
the allocation return address can be printed only for slab and slub,
and even then only when the necessary debug has been enabled.  For slab,
build with CONFIG_DEBUG_SLAB=y, and either use sizes with ample space
to the next power of two or use the SLAB_STORE_USER when creating the
kmem_cache structure.  For slub, build with CONFIG_SLUB_DEBUG=y and
boot with slub_debug=U, or pass SLAB_STORE_USER to kmem_cache_create()
if more focused use is desired.  Also for slub, use CONFIG_STACKTRACE
to enable printing of the allocation-time stack trace.

Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <linux-mm@kvack.org>
Reported-by: Andrii Nakryiko <andrii@kernel.org>
[ paulmck: Convert to printing and change names per Joonsoo Kim. ]
[ paulmck: Move slab definition per Stephen Rothwell and kbuild test robot. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 include/linux/mm.h   |  2 ++
 include/linux/slab.h |  2 ++
 mm/slab.c            | 28 +++++++++++++++++++++
 mm/slab.h            | 11 +++++++++
 mm/slab_common.c     | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 mm/slob.c            |  7 ++++++
 mm/slub.c            | 40 ++++++++++++++++++++++++++++++
 mm/util.c            | 25 +++++++++++++++++++
 8 files changed, 184 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ef360fe..1eea266 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3153,5 +3153,7 @@ unsigned long wp_shared_mapping_range(struct address_space *mapping,
 
 extern int sysctl_nr_trim_pages;
 
+void mem_dump_obj(void *object);
+
 #endif /* __KERNEL__ */
 #endif /* _LINUX_MM_H */
diff --git a/include/linux/slab.h b/include/linux/slab.h
index dd6897f..169b511 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -186,6 +186,8 @@ void kfree(const void *);
 void kfree_sensitive(const void *);
 size_t __ksize(const void *);
 size_t ksize(const void *);
+bool kmem_valid_obj(void *object);
+void kmem_dump_obj(void *object);
 
 #ifdef CONFIG_HAVE_HARDENED_USERCOPY_ALLOCATOR
 void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
diff --git a/mm/slab.c b/mm/slab.c
index b111356..72b6743 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3602,6 +3602,34 @@ void *kmem_cache_alloc_node_trace(struct kmem_cache *cachep,
 EXPORT_SYMBOL(kmem_cache_alloc_node_trace);
 #endif
 
+void kmem_provenance(struct kmem_provenance *kpp)
+{
+#ifdef DEBUG
+	struct kmem_cache *cachep;
+	void *object = kpp->kp_ptr;
+	unsigned int objnr;
+	void *objp;
+	struct page *page = kpp->kp_page;
+
+	cachep = page->slab_cache;
+	if (!(cachep->flags & SLAB_STORE_USER)) {
+		kpp->kp_ret = NULL;
+		goto nodebug;
+	}
+	objp = object - obj_offset(cachep);
+	page = virt_to_head_page(objp);
+	objnr = obj_to_index(cachep, page, objp);
+	objp = index_to_obj(cachep, page, objnr);
+	kpp->kp_objp = objp;
+	kpp->kp_ret = *dbg_userword(cachep, objp);
+nodebug:
+#else
+	kpp->kp_ret = NULL;
+#endif
+	if (kpp->kp_nstack)
+		kpp->kp_stack[0] = NULL;
+}
+
 static __always_inline void *
 __do_kmalloc_node(size_t size, gfp_t flags, int node, unsigned long caller)
 {
diff --git a/mm/slab.h b/mm/slab.h
index 6d7c6a5..28a41d5 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -630,4 +630,15 @@ static inline bool slab_want_init_on_free(struct kmem_cache *c)
 	return false;
 }
 
+#define KS_ADDRS_COUNT 16
+struct kmem_provenance {
+	void *kp_ptr;
+	struct page *kp_page;
+	void *kp_objp;
+	void *kp_ret;
+	void *kp_stack[KS_ADDRS_COUNT];
+	int kp_nstack;
+};
+void kmem_provenance(struct kmem_provenance *kpp);
+
 #endif /* MM_SLAB_H */
diff --git a/mm/slab_common.c b/mm/slab_common.c
index f9ccd5d..09f0cbc 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -536,6 +536,75 @@ bool slab_is_available(void)
 	return slab_state >= UP;
 }
 
+/**
+ * kmem_valid_obj - does the pointer reference a valid slab object?
+ * @object: pointer to query.
+ *
+ * Return: %true if the pointer is to a not-yet-freed object from
+ * kmalloc() or kmem_cache_alloc(), either %true or %false if the pointer
+ * is to an already-freed object, and %false otherwise.
+ */
+bool kmem_valid_obj(void *object)
+{
+	struct page *page;
+
+	if (!virt_addr_valid(object))
+		return false;
+	page = virt_to_head_page(object);
+	return PageSlab(page);
+}
+EXPORT_SYMBOL_GPL(kmem_valid_obj);
+
+/**
+ * kmem_dump_obj - Print available slab provenance information
+ * @object: slab object for which to find provenance information.
+ *
+ * This function uses pr_cont(), so that the caller is expected to have
+ * printed out whatever preamble is appropriate.  The provenance information
+ * depends on the type of object and on how much debugging is enabled.
+ * For a slab-cache object, the fact that it is a slab object is printed,
+ * and, if available, the slab name, return address, and stack trace from
+ * the allocation of that object.
+ *
+ * This function will splat if passed a pointer to a non-slab object.
+ * If you are not sure what type of object you have, you should instead
+ * use mem_dump_obj().
+ */
+void kmem_dump_obj(void *object)
+{
+	int i;
+	struct page *page;
+	struct kmem_provenance kp;
+
+	if (WARN_ON_ONCE(!virt_addr_valid(object)))
+		return;
+	page = virt_to_head_page(object);
+	if (WARN_ON_ONCE(!PageSlab(page))) {
+		pr_cont(" non-slab memory.\n");
+		return;
+	}
+	kp.kp_ptr = object;
+	kp.kp_page = page;
+	kp.kp_nstack = KS_ADDRS_COUNT;
+	kmem_provenance(&kp);
+	if (page->slab_cache)
+		pr_cont(" slab %s", page->slab_cache->name);
+	else
+		pr_cont(" slab ");
+	if (kp.kp_ret)
+		pr_cont(" allocated at %pS\n", kp.kp_ret);
+	else
+		pr_cont("\n");
+	if (kp.kp_stack[0]) {
+		for (i = 0; i < ARRAY_SIZE(kp.kp_stack); i++) {
+			if (!kp.kp_stack[i])
+				break;
+			pr_info("    %pS\n", kp.kp_stack[i]);
+		}
+	}
+}
+EXPORT_SYMBOL_GPL(kmem_dump_obj);
+
 #ifndef CONFIG_SLOB
 /* Create a cache during boot when no slab services are available yet */
 void __init create_boot_cache(struct kmem_cache *s, const char *name,
diff --git a/mm/slob.c b/mm/slob.c
index 7cc9805..fb10493 100644
--- a/mm/slob.c
+++ b/mm/slob.c
@@ -461,6 +461,13 @@ static void slob_free(void *block, int size)
 	spin_unlock_irqrestore(&slob_lock, flags);
 }
 
+void kmem_provenance(struct kmem_provenance *kpp)
+{
+	kpp->kp_ret = NULL;
+	if (kpp->kp_nstack)
+		kpp->kp_stack[0] = NULL;
+}
+
 /*
  * End of slob allocator proper. Begin kmem_cache_alloc and kmalloc frontend.
  */
diff --git a/mm/slub.c b/mm/slub.c
index b30be23..027fe0f 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3918,6 +3918,46 @@ int __kmem_cache_shutdown(struct kmem_cache *s)
 	return 0;
 }
 
+void kmem_provenance(struct kmem_provenance *kpp)
+{
+#ifdef CONFIG_SLUB_DEBUG
+	void *base;
+	int i;
+	void *object = kpp->kp_ptr;
+	unsigned int objnr;
+	void *objp;
+	struct page *page = kpp->kp_page;
+	struct kmem_cache *s = page->slab_cache;
+	struct track *trackp;
+
+	base = page_address(page);
+	objp = kasan_reset_tag(object);
+	objp = restore_red_left(s, objp);
+	objnr = obj_to_index(s, page, objp);
+	objp = base + s->size * objnr;
+	kpp->kp_objp = objp;
+	if (WARN_ON_ONCE(objp < base || objp >= base + page->objects * s->size || (objp - base) % s->size) ||
+	    !(s->flags & SLAB_STORE_USER))
+		goto nodebug;
+	trackp = get_track(s, objp, TRACK_ALLOC);
+	kpp->kp_ret = (void *)trackp->addr;
+#ifdef CONFIG_STACKTRACE
+	for (i = 0; i < kpp->kp_nstack && i < TRACK_ADDRS_COUNT; i++) {
+		kpp->kp_stack[i] = (void *)trackp->addrs[i];
+		if (!kpp->kp_stack[i])
+			break;
+	}
+#endif
+	if (kpp->kp_stack && i < kpp->kp_nstack)
+		kpp->kp_stack[i] = NULL;
+	return;
+nodebug:
+#endif
+	kpp->kp_ret = NULL;
+	if (kpp->kp_nstack)
+		kpp->kp_stack[0] = NULL;
+}
+
 /********************************************************************
  *		Kmalloc subsystem
  *******************************************************************/
diff --git a/mm/util.c b/mm/util.c
index 4ddb6e1..d0e60d2 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -970,3 +970,28 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
 	kunmap_atomic(addr1);
 	return ret;
 }
+
+/**
+ * mem_dump_obj - Print available provenance information
+ * @object: object for which to find provenance information.
+ *
+ * This function uses pr_cont(), so that the caller is expected to have
+ * printed out whatever preamble is appropriate.  The provenance information
+ * depends on the type of object and on how much debugging is enabled.
+ * For example, for a slab-cache object, the slab name is printed, and,
+ * if available, the return address and stack trace from the allocation
+ * of that object.
+ */
+void mem_dump_obj(void *object)
+{
+	if (!virt_addr_valid(object)) {
+		pr_cont(" non-paged (local) memory.\n");
+		return;
+	}
+	if (kmem_valid_obj(object)) {
+		kmem_dump_obj(object);
+		return;
+	}
+	pr_cont(" non-slab memory.\n");
+}
+EXPORT_SYMBOL_GPL(mem_dump_obj);
-- 
2.9.5

Re: [PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[Christoph Hellwig]

Your two new exports don't actually seem to get used in modular code
at all in this series.

Re: [PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[Paul E. McKenney]

On Wed, Dec 09, 2020 at 08:17:10AM +0000, Christoph Hellwig wrote:
> Your two new exports don't actually seem to get used in modular code
> at all in this series.

Indeed, and I either need to remove the exports or make my test code in
kernel/rcu/rcuscale.o suitable for upstreaming.  Or find the appropriate
mm/slab selftest location.

							Thanx, Paul

Re: [PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[Christoph Hellwig]

On Wed, Dec 09, 2020 at 06:57:02AM -0800, Paul E. McKenney wrote:
> On Wed, Dec 09, 2020 at 08:17:10AM +0000, Christoph Hellwig wrote:
> > Your two new exports don't actually seem to get used in modular code
> > at all in this series.
> 
> Indeed, and I either need to remove the exports or make my test code in
> kernel/rcu/rcuscale.o suitable for upstreaming.  Or find the appropriate
> mm/slab selftest location.

I'd rather not export something like this which pokes deep into
internals.  That being said I've been working on off on a
EXPORT_SYMBOL_FOR() that just exports a symbol to one specific module.
Hopefully I'll finish it for the next merge window, and with that
I'd feel much more comfortable with an export.

Re: [PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[Paul E. McKenney]

On Wed, Dec 09, 2020 at 05:53:06PM +0000, Christoph Hellwig wrote:
> On Wed, Dec 09, 2020 at 06:57:02AM -0800, Paul E. McKenney wrote:
> > On Wed, Dec 09, 2020 at 08:17:10AM +0000, Christoph Hellwig wrote:
> > > Your two new exports don't actually seem to get used in modular code
> > > at all in this series.
> > 
> > Indeed, and I either need to remove the exports or make my test code in
> > kernel/rcu/rcuscale.o suitable for upstreaming.  Or find the appropriate
> > mm/slab selftest location.
> 
> I'd rather not export something like this which pokes deep into
> internals.  That being said I've been working on off on a
> EXPORT_SYMBOL_FOR() that just exports a symbol to one specific module.
> Hopefully I'll finish it for the next merge window, and with that
> I'd feel much more comfortable with an export.

That would be really useful!  I have a number of symbols that should
only be used by a few specific in-tree modules, independent of this
patch series.

For my part, I will see if there is a good mm-related location for this
sort of selftest.

							Thanx, Paul

Re: [PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[Vlastimil Babka]

On 12/9/20 2:12 AM, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> There are kernel facilities such as per-CPU reference counts that give
> error messages in generic handlers or callbacks, whose messages are
> unenlightening.  In the case of per-CPU reference-count underflow, this
> is not a problem when creating a new use of this facility because in that
> case the bug is almost certainly in the code implementing that new use.
> However, trouble arises when deploying across many systems, which might
> exercise corner cases that were not seen during development and testing.
> Here, it would be really nice to get some kind of hint as to which of
> several uses the underflow was caused by.
> 
> This commit therefore exposes a mem_dump_obj() function that takes
> a pointer to memory (which must still be allocated if it has been
> dynamically allocated) and prints available information on where that
> memory came from.  This pointer can reference the middle of the block as
> well as the beginning of the block, as needed by things like RCU callback
> functions and timer handlers that might not know where the beginning of
> the memory block is.  These functions and handlers can use mem_dump_obj()
> to print out better hints as to where the problem might lie.

Sounds useful, yeah. It occured to me at least once that we don't have a nice
generic way to print this kind of info. I usually dig it from a crash dump...

> The information printed can depend on kernel configuration.  For example,
> the allocation return address can be printed only for slab and slub,
> and even then only when the necessary debug has been enabled.  For slab,
> build with CONFIG_DEBUG_SLAB=y, and either use sizes with ample space
> to the next power of two or use the SLAB_STORE_USER when creating the
> kmem_cache structure.  For slub, build with CONFIG_SLUB_DEBUG=y and
> boot with slub_debug=U, or pass SLAB_STORE_USER to kmem_cache_create()
> if more focused use is desired.  Also for slub, use CONFIG_STACKTRACE
> to enable printing of the allocation-time stack trace.
> 
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: <linux-mm@kvack.org>
> Reported-by: Andrii Nakryiko <andrii@kernel.org>
> [ paulmck: Convert to printing and change names per Joonsoo Kim. ]
> [ paulmck: Move slab definition per Stephen Rothwell and kbuild test robot. ]
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

...

> +/**
> + * kmem_valid_obj - does the pointer reference a valid slab object?
> + * @object: pointer to query.
> + *
> + * Return: %true if the pointer is to a not-yet-freed object from
> + * kmalloc() or kmem_cache_alloc(), either %true or %false if the pointer
> + * is to an already-freed object, and %false otherwise.
> + */

It should be possible to find out more about object being free or not, than you
currently do. At least to find out if it's definitely free. When it appears
allocated, it can be actually still free in some kind of e.g. per-cpu or
per-node cache that would be infeasible to check. But that improvement to the
output can be also added later. Also SLUB stores the freeing stacktrace, which
might be useful...

> +bool kmem_valid_obj(void *object)
> +{
> +	struct page *page;
> +
> +	if (!virt_addr_valid(object))
> +		return false;
> +	page = virt_to_head_page(object);
> +	return PageSlab(page);
> +}
> +EXPORT_SYMBOL_GPL(kmem_valid_obj);
> +
> +/**
> + * kmem_dump_obj - Print available slab provenance information
> + * @object: slab object for which to find provenance information.
> + *
> + * This function uses pr_cont(), so that the caller is expected to have
> + * printed out whatever preamble is appropriate.  The provenance information
> + * depends on the type of object and on how much debugging is enabled.
> + * For a slab-cache object, the fact that it is a slab object is printed,
> + * and, if available, the slab name, return address, and stack trace from
> + * the allocation of that object.
> + *
> + * This function will splat if passed a pointer to a non-slab object.
> + * If you are not sure what type of object you have, you should instead
> + * use mem_dump_obj().
> + */
> +void kmem_dump_obj(void *object)
> +{
> +	int i;
> +	struct page *page;
> +	struct kmem_provenance kp;
> +
> +	if (WARN_ON_ONCE(!virt_addr_valid(object)))
> +		return;
> +	page = virt_to_head_page(object);
> +	if (WARN_ON_ONCE(!PageSlab(page))) {
> +		pr_cont(" non-slab memory.\n");
> +		return;
> +	}
> +	kp.kp_ptr = object;
> +	kp.kp_page = page;
> +	kp.kp_nstack = KS_ADDRS_COUNT;
> +	kmem_provenance(&kp);

You don't seem to be printing kp.kp_objp anywhere? (unless in later patch, but
would make sense in this patch already).

> +	if (page->slab_cache)
> +		pr_cont(" slab %s", page->slab_cache->name);
> +	else
> +		pr_cont(" slab ");
> +	if (kp.kp_ret)
> +		pr_cont(" allocated at %pS\n", kp.kp_ret);
> +	else
> +		pr_cont("\n");
> +	if (kp.kp_stack[0]) {
> +		for (i = 0; i < ARRAY_SIZE(kp.kp_stack); i++) {
> +			if (!kp.kp_stack[i])
> +				break;
> +			pr_info("    %pS\n", kp.kp_stack[i]);
> +		}
> +	}
> +}

...

> diff --git a/mm/slub.c b/mm/slub.c
> index b30be23..027fe0f 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3918,6 +3918,46 @@ int __kmem_cache_shutdown(struct kmem_cache *s)
>  	return 0;
>  }
>  
> +void kmem_provenance(struct kmem_provenance *kpp)
> +{
> +#ifdef CONFIG_SLUB_DEBUG

I'd expect at least the very basic stuff (kp_obj) to be possible to determine
even under !CONFIG_SLUB_DEBUG?

> +	void *base;
> +	int i;
> +	void *object = kpp->kp_ptr;
> +	unsigned int objnr;
> +	void *objp;
> +	struct page *page = kpp->kp_page;
> +	struct kmem_cache *s = page->slab_cache;
> +	struct track *trackp;
> +
> +	base = page_address(page);
> +	objp = kasan_reset_tag(object);
> +	objp = restore_red_left(s, objp);
> +	objnr = obj_to_index(s, page, objp);
> +	objp = base + s->size * objnr;
> +	kpp->kp_objp = objp;
> +	if (WARN_ON_ONCE(objp < base || objp >= base + page->objects * s->size || (objp - base) % s->size) ||
> +	    !(s->flags & SLAB_STORE_USER))
> +		goto nodebug;
> +	trackp = get_track(s, objp, TRACK_ALLOC);
> +	kpp->kp_ret = (void *)trackp->addr;
> +#ifdef CONFIG_STACKTRACE
> +	for (i = 0; i < kpp->kp_nstack && i < TRACK_ADDRS_COUNT; i++) {
> +		kpp->kp_stack[i] = (void *)trackp->addrs[i];
> +		if (!kpp->kp_stack[i])
> +			break;
> +	}
> +#endif
> +	if (kpp->kp_stack && i < kpp->kp_nstack)
> +		kpp->kp_stack[i] = NULL;
> +	return;
> +nodebug:
> +#endif
> +	kpp->kp_ret = NULL;
> +	if (kpp->kp_nstack)
> +		kpp->kp_stack[0] = NULL;
> +}
> +
>  /********************************************************************
>   *		Kmalloc subsystem
>   *******************************************************************/
> diff --git a/mm/util.c b/mm/util.c
> index 4ddb6e1..d0e60d2 100644
> --- a/mm/util.c
> +++ b/mm/util.c

I think mm/debug.c is a better fit as it already has dump_page() of a similar
nature. Also you can call that from from mem_dump_obj() at least in case when
the more specific handlers fail. It will even include page_owner info if enabled! :)

Thanks,
Vlastimil

> @@ -970,3 +970,28 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
>  	kunmap_atomic(addr1);
>  	return ret;
>  }
> +
> +/**
> + * mem_dump_obj - Print available provenance information
> + * @object: object for which to find provenance information.
> + *
> + * This function uses pr_cont(), so that the caller is expected to have
> + * printed out whatever preamble is appropriate.  The provenance information
> + * depends on the type of object and on how much debugging is enabled.
> + * For example, for a slab-cache object, the slab name is printed, and,
> + * if available, the return address and stack trace from the allocation
> + * of that object.
> + */
> +void mem_dump_obj(void *object)
> +{
> +	if (!virt_addr_valid(object)) {
> +		pr_cont(" non-paged (local) memory.\n");
> +		return;
> +	}
> +	if (kmem_valid_obj(object)) {
> +		kmem_dump_obj(object);
> +		return;
> +	}
> +	pr_cont(" non-slab memory.\n");
> +}
> +EXPORT_SYMBOL_GPL(mem_dump_obj);
> 

Re: [PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[Paul E. McKenney]

On Wed, Dec 09, 2020 at 06:28:50PM +0100, Vlastimil Babka wrote:
> On 12/9/20 2:12 AM, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> > 
> > There are kernel facilities such as per-CPU reference counts that give
> > error messages in generic handlers or callbacks, whose messages are
> > unenlightening.  In the case of per-CPU reference-count underflow, this
> > is not a problem when creating a new use of this facility because in that
> > case the bug is almost certainly in the code implementing that new use.
> > However, trouble arises when deploying across many systems, which might
> > exercise corner cases that were not seen during development and testing.
> > Here, it would be really nice to get some kind of hint as to which of
> > several uses the underflow was caused by.
> > 
> > This commit therefore exposes a mem_dump_obj() function that takes
> > a pointer to memory (which must still be allocated if it has been
> > dynamically allocated) and prints available information on where that
> > memory came from.  This pointer can reference the middle of the block as
> > well as the beginning of the block, as needed by things like RCU callback
> > functions and timer handlers that might not know where the beginning of
> > the memory block is.  These functions and handlers can use mem_dump_obj()
> > to print out better hints as to where the problem might lie.
> 
> Sounds useful, yeah. It occured to me at least once that we don't have a nice
> generic way to print this kind of info. I usually dig it from a crash dump...

Glad to hear that it might be helpful, and thank you for looking this
over!

> > The information printed can depend on kernel configuration.  For example,
> > the allocation return address can be printed only for slab and slub,
> > and even then only when the necessary debug has been enabled.  For slab,
> > build with CONFIG_DEBUG_SLAB=y, and either use sizes with ample space
> > to the next power of two or use the SLAB_STORE_USER when creating the
> > kmem_cache structure.  For slub, build with CONFIG_SLUB_DEBUG=y and
> > boot with slub_debug=U, or pass SLAB_STORE_USER to kmem_cache_create()
> > if more focused use is desired.  Also for slub, use CONFIG_STACKTRACE
> > to enable printing of the allocation-time stack trace.
> > 
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Pekka Enberg <penberg@kernel.org>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: <linux-mm@kvack.org>
> > Reported-by: Andrii Nakryiko <andrii@kernel.org>
> > [ paulmck: Convert to printing and change names per Joonsoo Kim. ]
> > [ paulmck: Move slab definition per Stephen Rothwell and kbuild test robot. ]
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> ...
> 
> > +/**
> > + * kmem_valid_obj - does the pointer reference a valid slab object?
> > + * @object: pointer to query.
> > + *
> > + * Return: %true if the pointer is to a not-yet-freed object from
> > + * kmalloc() or kmem_cache_alloc(), either %true or %false if the pointer
> > + * is to an already-freed object, and %false otherwise.
> > + */
> 
> It should be possible to find out more about object being free or not, than you
> currently do. At least to find out if it's definitely free. When it appears
> allocated, it can be actually still free in some kind of e.g. per-cpu or
> per-node cache that would be infeasible to check. But that improvement to the
> output can be also added later. Also SLUB stores the freeing stacktrace, which
> might be useful...

I can see how this could help debugging a use-after-free situation,
at least as long as the poor sap that subsequently allocated it doesn't
free it.

I can easily add more fields to the kmem_provenance structure.  Maybe
it would make sense to have another exported API that you provide a
kmem_provenance structure to, and it fills it in.

One caution though...  I rely on the object being allocated.
If it officially might already be freed, complex and high-overhead
synchronization seems to be required to safely access the various data
structures.

So any use on an already-freed object is on a "you break it you get to
keep the pieces" basis.  On the other hand, if you are dealing with a
use-after-free situation, life is hard anyway.

Or am I missing your point?

> > +bool kmem_valid_obj(void *object)
> > +{
> > +	struct page *page;
> > +
> > +	if (!virt_addr_valid(object))
> > +		return false;
> > +	page = virt_to_head_page(object);
> > +	return PageSlab(page);
> > +}
> > +EXPORT_SYMBOL_GPL(kmem_valid_obj);
> > +
> > +/**
> > + * kmem_dump_obj - Print available slab provenance information
> > + * @object: slab object for which to find provenance information.
> > + *
> > + * This function uses pr_cont(), so that the caller is expected to have
> > + * printed out whatever preamble is appropriate.  The provenance information
> > + * depends on the type of object and on how much debugging is enabled.
> > + * For a slab-cache object, the fact that it is a slab object is printed,
> > + * and, if available, the slab name, return address, and stack trace from
> > + * the allocation of that object.
> > + *
> > + * This function will splat if passed a pointer to a non-slab object.
> > + * If you are not sure what type of object you have, you should instead
> > + * use mem_dump_obj().
> > + */
> > +void kmem_dump_obj(void *object)
> > +{
> > +	int i;
> > +	struct page *page;
> > +	struct kmem_provenance kp;
> > +
> > +	if (WARN_ON_ONCE(!virt_addr_valid(object)))
> > +		return;
> > +	page = virt_to_head_page(object);
> > +	if (WARN_ON_ONCE(!PageSlab(page))) {
> > +		pr_cont(" non-slab memory.\n");
> > +		return;
> > +	}
> > +	kp.kp_ptr = object;
> > +	kp.kp_page = page;
> > +	kp.kp_nstack = KS_ADDRS_COUNT;
> > +	kmem_provenance(&kp);
> 
> You don't seem to be printing kp.kp_objp anywhere? (unless in later patch, but
> would make sense in this patch already).

Good point!

However, please note that the various debugging options that reserve
space at the beginning.  This can make the meaning of kp.kp_objp a bit
different than one might expect.

> > +	if (page->slab_cache)
> > +		pr_cont(" slab %s", page->slab_cache->name);
> > +	else
> > +		pr_cont(" slab ");
> > +	if (kp.kp_ret)
> > +		pr_cont(" allocated at %pS\n", kp.kp_ret);
> > +	else
> > +		pr_cont("\n");
> > +	if (kp.kp_stack[0]) {
> > +		for (i = 0; i < ARRAY_SIZE(kp.kp_stack); i++) {
> > +			if (!kp.kp_stack[i])
> > +				break;
> > +			pr_info("    %pS\n", kp.kp_stack[i]);
> > +		}
> > +	}
> > +}
> 
> ...
> 
> > diff --git a/mm/slub.c b/mm/slub.c
> > index b30be23..027fe0f 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -3918,6 +3918,46 @@ int __kmem_cache_shutdown(struct kmem_cache *s)
> >  	return 0;
> >  }
> >  
> > +void kmem_provenance(struct kmem_provenance *kpp)
> > +{
> > +#ifdef CONFIG_SLUB_DEBUG
> 
> I'd expect at least the very basic stuff (kp_obj) to be possible to determine
> even under !CONFIG_SLUB_DEBUG?

And doing it that way even saves a line of code!  ;-)

> > +	void *base;
> > +	int i;
> > +	void *object = kpp->kp_ptr;
> > +	unsigned int objnr;
> > +	void *objp;
> > +	struct page *page = kpp->kp_page;
> > +	struct kmem_cache *s = page->slab_cache;
> > +	struct track *trackp;
> > +
> > +	base = page_address(page);
> > +	objp = kasan_reset_tag(object);
> > +	objp = restore_red_left(s, objp);
> > +	objnr = obj_to_index(s, page, objp);
> > +	objp = base + s->size * objnr;
> > +	kpp->kp_objp = objp;
> > +	if (WARN_ON_ONCE(objp < base || objp >= base + page->objects * s->size || (objp - base) % s->size) ||
> > +	    !(s->flags & SLAB_STORE_USER))
> > +		goto nodebug;
> > +	trackp = get_track(s, objp, TRACK_ALLOC);
> > +	kpp->kp_ret = (void *)trackp->addr;
> > +#ifdef CONFIG_STACKTRACE
> > +	for (i = 0; i < kpp->kp_nstack && i < TRACK_ADDRS_COUNT; i++) {
> > +		kpp->kp_stack[i] = (void *)trackp->addrs[i];
> > +		if (!kpp->kp_stack[i])
> > +			break;
> > +	}
> > +#endif
> > +	if (kpp->kp_stack && i < kpp->kp_nstack)
> > +		kpp->kp_stack[i] = NULL;
> > +	return;
> > +nodebug:
> > +#endif
> > +	kpp->kp_ret = NULL;
> > +	if (kpp->kp_nstack)
> > +		kpp->kp_stack[0] = NULL;
> > +}
> > +
> >  /********************************************************************
> >   *		Kmalloc subsystem
> >   *******************************************************************/
> > diff --git a/mm/util.c b/mm/util.c
> > index 4ddb6e1..d0e60d2 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> 
> I think mm/debug.c is a better fit as it already has dump_page() of a similar
> nature. Also you can call that from from mem_dump_obj() at least in case when
> the more specific handlers fail. It will even include page_owner info if enabled! :)

I will count this as one vote for mm/debug.c.

Two things to consider, though...  First, Joonsoo suggests that the fact
that this produces useful information without any debugging information
enabled makes it not be debugging as such.  Second, mm/debug.c does
not include either slab.h or vmalloc.h.  The second might not be a
showstopper, but I was interpreting this to mean that its role was
less central.

							Thanx, Paul

> Thanks,
> Vlastimil
> 
> > @@ -970,3 +970,28 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
> >  	kunmap_atomic(addr1);
> >  	return ret;
> >  }
> > +
> > +/**
> > + * mem_dump_obj - Print available provenance information
> > + * @object: object for which to find provenance information.
> > + *
> > + * This function uses pr_cont(), so that the caller is expected to have
> > + * printed out whatever preamble is appropriate.  The provenance information
> > + * depends on the type of object and on how much debugging is enabled.
> > + * For example, for a slab-cache object, the slab name is printed, and,
> > + * if available, the return address and stack trace from the allocation
> > + * of that object.
> > + */
> > +void mem_dump_obj(void *object)
> > +{
> > +	if (!virt_addr_valid(object)) {
> > +		pr_cont(" non-paged (local) memory.\n");
> > +		return;
> > +	}
> > +	if (kmem_valid_obj(object)) {
> > +		kmem_dump_obj(object);
> > +		return;
> > +	}
> > +	pr_cont(" non-slab memory.\n");
> > +}
> > +EXPORT_SYMBOL_GPL(mem_dump_obj);
> > 
> 

Re: [PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[Vlastimil Babka]

On 12/10/20 12:04 AM, Paul E. McKenney wrote:
>> > +/**
>> > + * kmem_valid_obj - does the pointer reference a valid slab object?
>> > + * @object: pointer to query.
>> > + *
>> > + * Return: %true if the pointer is to a not-yet-freed object from
>> > + * kmalloc() or kmem_cache_alloc(), either %true or %false if the pointer
>> > + * is to an already-freed object, and %false otherwise.
>> > + */
>> 
>> It should be possible to find out more about object being free or not, than you
>> currently do. At least to find out if it's definitely free. When it appears
>> allocated, it can be actually still free in some kind of e.g. per-cpu or
>> per-node cache that would be infeasible to check. But that improvement to the
>> output can be also added later. Also SLUB stores the freeing stacktrace, which
>> might be useful...
> 
> I can see how this could help debugging a use-after-free situation,
> at least as long as the poor sap that subsequently allocated it doesn't
> free it.
> 
> I can easily add more fields to the kmem_provenance structure.  Maybe
> it would make sense to have another exported API that you provide a
> kmem_provenance structure to, and it fills it in.
> 
> One caution though...  I rely on the object being allocated.
> If it officially might already be freed, complex and high-overhead
> synchronization seems to be required to safely access the various data
> structures.

Good point! It's easy to forget that when being used to similar digging in a
crash dump, where nothing changes.

> So any use on an already-freed object is on a "you break it you get to
> keep the pieces" basis.  On the other hand, if you are dealing with a
> use-after-free situation, life is hard anyway.

Yeah, even now I think it's potentially dangerous, as you can get
kmem_valid_obj() as true because PageSlab(page) is true. But the object might be
already free, so as soon as another CPU frees another object from the same slab
page, the page gets also freed... or it was already freed and then allocated by
another slab so it's PageSlab() again.
I guess at least some safety could be achieved by pinning the page with
get_page_unless_zero. But maybe your current implementation is already safe,
need to check in detail.

> Or am I missing your point?
> 
>> > +bool kmem_valid_obj(void *object)
>> > +{
>> > +	struct page *page;
>> > +
>> > +	if (!virt_addr_valid(object))
>> > +		return false;
>> > +	page = virt_to_head_page(object);
>> > +	return PageSlab(page);
>> > +}
>> > +EXPORT_SYMBOL_GPL(kmem_valid_obj);
>> > +
>> > +/**
>> > + * kmem_dump_obj - Print available slab provenance information
>> > + * @object: slab object for which to find provenance information.
>> > + *
>> > + * This function uses pr_cont(), so that the caller is expected to have
>> > + * printed out whatever preamble is appropriate.  The provenance information
>> > + * depends on the type of object and on how much debugging is enabled.
>> > + * For a slab-cache object, the fact that it is a slab object is printed,
>> > + * and, if available, the slab name, return address, and stack trace from
>> > + * the allocation of that object.
>> > + *
>> > + * This function will splat if passed a pointer to a non-slab object.
>> > + * If you are not sure what type of object you have, you should instead
>> > + * use mem_dump_obj().
>> > + */
>> > +void kmem_dump_obj(void *object)
>> > +{
>> > +	int i;
>> > +	struct page *page;
>> > +	struct kmem_provenance kp;
>> > +
>> > +	if (WARN_ON_ONCE(!virt_addr_valid(object)))
>> > +		return;
>> > +	page = virt_to_head_page(object);
>> > +	if (WARN_ON_ONCE(!PageSlab(page))) {
>> > +		pr_cont(" non-slab memory.\n");
>> > +		return;
>> > +	}
>> > +	kp.kp_ptr = object;
>> > +	kp.kp_page = page;
>> > +	kp.kp_nstack = KS_ADDRS_COUNT;
>> > +	kmem_provenance(&kp);
>> 
>> You don't seem to be printing kp.kp_objp anywhere? (unless in later patch, but
>> would make sense in this patch already).
> 
> Good point!
> 
> However, please note that the various debugging options that reserve
> space at the beginning.  This can make the meaning of kp.kp_objp a bit
> different than one might expect.

Yeah, I think the best would be to match the address that
kmalloc/kmem_cache_alloc() would return, thus the beginning of the object
itself, so you can calculate the offset within it, etc.

>> > --- a/mm/util.c
>> > +++ b/mm/util.c
>> 
>> I think mm/debug.c is a better fit as it already has dump_page() of a similar
>> nature. Also you can call that from from mem_dump_obj() at least in case when
>> the more specific handlers fail. It will even include page_owner info if enabled! :)
> 
> I will count this as one vote for mm/debug.c.
> 
> Two things to consider, though...  First, Joonsoo suggests that the fact
> that this produces useful information without any debugging information
> enabled makes it not be debugging as such.

Well there's already dump_page() which also produces information without special
configs.
We're not the best subsystem in this kind of consistency...

> Second, mm/debug.c does
> not include either slab.h or vmalloc.h.  The second might not be a
> showstopper, but I was interpreting this to mean that its role was
> less central.

I think it can include whatever becomes needed there :)

> 							Thanx, Paul
> 
>> Thanks,
>> Vlastimil
>> 
>> > @@ -970,3 +970,28 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
>> >  	kunmap_atomic(addr1);
>> >  	return ret;
>> >  }
>> > +
>> > +/**
>> > + * mem_dump_obj - Print available provenance information
>> > + * @object: object for which to find provenance information.
>> > + *
>> > + * This function uses pr_cont(), so that the caller is expected to have
>> > + * printed out whatever preamble is appropriate.  The provenance information
>> > + * depends on the type of object and on how much debugging is enabled.
>> > + * For example, for a slab-cache object, the slab name is printed, and,
>> > + * if available, the return address and stack trace from the allocation
>> > + * of that object.
>> > + */
>> > +void mem_dump_obj(void *object)
>> > +{
>> > +	if (!virt_addr_valid(object)) {
>> > +		pr_cont(" non-paged (local) memory.\n");
>> > +		return;
>> > +	}
>> > +	if (kmem_valid_obj(object)) {
>> > +		kmem_dump_obj(object);
>> > +		return;
>> > +	}
>> > +	pr_cont(" non-slab memory.\n");
>> > +}
>> > +EXPORT_SYMBOL_GPL(mem_dump_obj);
>> > 
>> 
> 

Re: [PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[Paul E. McKenney]

On Thu, Dec 10, 2020 at 11:48:26AM +0100, Vlastimil Babka wrote:
> On 12/10/20 12:04 AM, Paul E. McKenney wrote:
> >> > +/**
> >> > + * kmem_valid_obj - does the pointer reference a valid slab object?
> >> > + * @object: pointer to query.
> >> > + *
> >> > + * Return: %true if the pointer is to a not-yet-freed object from
> >> > + * kmalloc() or kmem_cache_alloc(), either %true or %false if the pointer
> >> > + * is to an already-freed object, and %false otherwise.
> >> > + */
> >> 
> >> It should be possible to find out more about object being free or not, than you
> >> currently do. At least to find out if it's definitely free. When it appears
> >> allocated, it can be actually still free in some kind of e.g. per-cpu or
> >> per-node cache that would be infeasible to check. But that improvement to the
> >> output can be also added later. Also SLUB stores the freeing stacktrace, which
> >> might be useful...
> > 
> > I can see how this could help debugging a use-after-free situation,
> > at least as long as the poor sap that subsequently allocated it doesn't
> > free it.
> > 
> > I can easily add more fields to the kmem_provenance structure.  Maybe
> > it would make sense to have another exported API that you provide a
> > kmem_provenance structure to, and it fills it in.
> > 
> > One caution though...  I rely on the object being allocated.
> > If it officially might already be freed, complex and high-overhead
> > synchronization seems to be required to safely access the various data
> > structures.
> 
> Good point! It's easy to forget that when being used to similar digging in a
> crash dump, where nothing changes.

Maybe a similar addition to the crash-analysis tools would be helpful?

> > So any use on an already-freed object is on a "you break it you get to
> > keep the pieces" basis.  On the other hand, if you are dealing with a
> > use-after-free situation, life is hard anyway.
> 
> Yeah, even now I think it's potentially dangerous, as you can get
> kmem_valid_obj() as true because PageSlab(page) is true. But the object might be
> already free, so as soon as another CPU frees another object from the same slab
> page, the page gets also freed... or it was already freed and then allocated by
> another slab so it's PageSlab() again.
> I guess at least some safety could be achieved by pinning the page with
> get_page_unless_zero. But maybe your current implementation is already safe,
> need to check in detail.

The code on the various free paths looks to me to make the same
assumptions that I am making.  So if this is unsafe, we have other
problems.

> > Or am I missing your point?
> > 
> >> > +bool kmem_valid_obj(void *object)
> >> > +{
> >> > +	struct page *page;
> >> > +
> >> > +	if (!virt_addr_valid(object))
> >> > +		return false;
> >> > +	page = virt_to_head_page(object);
> >> > +	return PageSlab(page);
> >> > +}
> >> > +EXPORT_SYMBOL_GPL(kmem_valid_obj);
> >> > +
> >> > +/**
> >> > + * kmem_dump_obj - Print available slab provenance information
> >> > + * @object: slab object for which to find provenance information.
> >> > + *
> >> > + * This function uses pr_cont(), so that the caller is expected to have
> >> > + * printed out whatever preamble is appropriate.  The provenance information
> >> > + * depends on the type of object and on how much debugging is enabled.
> >> > + * For a slab-cache object, the fact that it is a slab object is printed,
> >> > + * and, if available, the slab name, return address, and stack trace from
> >> > + * the allocation of that object.
> >> > + *
> >> > + * This function will splat if passed a pointer to a non-slab object.
> >> > + * If you are not sure what type of object you have, you should instead
> >> > + * use mem_dump_obj().
> >> > + */
> >> > +void kmem_dump_obj(void *object)
> >> > +{
> >> > +	int i;
> >> > +	struct page *page;
> >> > +	struct kmem_provenance kp;
> >> > +
> >> > +	if (WARN_ON_ONCE(!virt_addr_valid(object)))
> >> > +		return;
> >> > +	page = virt_to_head_page(object);
> >> > +	if (WARN_ON_ONCE(!PageSlab(page))) {
> >> > +		pr_cont(" non-slab memory.\n");
> >> > +		return;
> >> > +	}
> >> > +	kp.kp_ptr = object;
> >> > +	kp.kp_page = page;
> >> > +	kp.kp_nstack = KS_ADDRS_COUNT;
> >> > +	kmem_provenance(&kp);
> >> 
> >> You don't seem to be printing kp.kp_objp anywhere? (unless in later patch, but
> >> would make sense in this patch already).
> > 
> > Good point!
> > 
> > However, please note that the various debugging options that reserve
> > space at the beginning.  This can make the meaning of kp.kp_objp a bit
> > different than one might expect.
> 
> Yeah, I think the best would be to match the address that
> kmalloc/kmem_cache_alloc() would return, thus the beginning of the object
> itself, so you can calculate the offset within it, etc.

My thought is to do both.  Show the start address, the data offset (if
nonzero), and the pointer offset within the data.  My guess is that in
the absence of things like slub_debug=U, the pointer offset within the
data is the best way to figure out which structure is involved.

Or do you use other tricks to work this sort of thing out?

> >> > --- a/mm/util.c
> >> > +++ b/mm/util.c
> >> 
> >> I think mm/debug.c is a better fit as it already has dump_page() of a similar
> >> nature. Also you can call that from from mem_dump_obj() at least in case when
> >> the more specific handlers fail. It will even include page_owner info if enabled! :)
> > 
> > I will count this as one vote for mm/debug.c.
> > 
> > Two things to consider, though...  First, Joonsoo suggests that the fact
> > that this produces useful information without any debugging information
> > enabled makes it not be debugging as such.
> 
> Well there's already dump_page() which also produces information without special
> configs.
> We're not the best subsystem in this kind of consistency...
> 
> > Second, mm/debug.c does
> > not include either slab.h or vmalloc.h.  The second might not be a
> > showstopper, but I was interpreting this to mean that its role was
> > less central.
> 
> I think it can include whatever becomes needed there :)

I figured that there was a significant probability that I would have to
move it, and I really don't have a basis for a preference, let alone
a preference.  But I would like to avoid moving it more than once, so
I also figured I should give anyone else having an educated preference
a chance to speak up.  ;-)

							Thanx, Paul

> >> Thanks,
> >> Vlastimil
> >> 
> >> > @@ -970,3 +970,28 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
> >> >  	kunmap_atomic(addr1);
> >> >  	return ret;
> >> >  }
> >> > +
> >> > +/**
> >> > + * mem_dump_obj - Print available provenance information
> >> > + * @object: object for which to find provenance information.
> >> > + *
> >> > + * This function uses pr_cont(), so that the caller is expected to have
> >> > + * printed out whatever preamble is appropriate.  The provenance information
> >> > + * depends on the type of object and on how much debugging is enabled.
> >> > + * For example, for a slab-cache object, the slab name is printed, and,
> >> > + * if available, the return address and stack trace from the allocation
> >> > + * of that object.
> >> > + */
> >> > +void mem_dump_obj(void *object)
> >> > +{
> >> > +	if (!virt_addr_valid(object)) {
> >> > +		pr_cont(" non-paged (local) memory.\n");
> >> > +		return;
> >> > +	}
> >> > +	if (kmem_valid_obj(object)) {
> >> > +		kmem_dump_obj(object);
> >> > +		return;
> >> > +	}
> >> > +	pr_cont(" non-slab memory.\n");
> >> > +}
> >> > +EXPORT_SYMBOL_GPL(mem_dump_obj);
> >> > 
> >> 
> > 
> 

Re: [PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[Joonsoo Kim]

On Tue, Dec 08, 2020 at 05:12:59PM -0800, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> There are kernel facilities such as per-CPU reference counts that give
> error messages in generic handlers or callbacks, whose messages are
> unenlightening.  In the case of per-CPU reference-count underflow, this
> is not a problem when creating a new use of this facility because in that
> case the bug is almost certainly in the code implementing that new use.
> However, trouble arises when deploying across many systems, which might
> exercise corner cases that were not seen during development and testing.
> Here, it would be really nice to get some kind of hint as to which of
> several uses the underflow was caused by.
> 
> This commit therefore exposes a mem_dump_obj() function that takes
> a pointer to memory (which must still be allocated if it has been
> dynamically allocated) and prints available information on where that
> memory came from.  This pointer can reference the middle of the block as
> well as the beginning of the block, as needed by things like RCU callback
> functions and timer handlers that might not know where the beginning of
> the memory block is.  These functions and handlers can use mem_dump_obj()
> to print out better hints as to where the problem might lie.
> 
> The information printed can depend on kernel configuration.  For example,
> the allocation return address can be printed only for slab and slub,
> and even then only when the necessary debug has been enabled.  For slab,
> build with CONFIG_DEBUG_SLAB=y, and either use sizes with ample space
> to the next power of two or use the SLAB_STORE_USER when creating the
> kmem_cache structure.  For slub, build with CONFIG_SLUB_DEBUG=y and
> boot with slub_debug=U, or pass SLAB_STORE_USER to kmem_cache_create()
> if more focused use is desired.  Also for slub, use CONFIG_STACKTRACE
> to enable printing of the allocation-time stack trace.
> 
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: <linux-mm@kvack.org>
> Reported-by: Andrii Nakryiko <andrii@kernel.org>
> [ paulmck: Convert to printing and change names per Joonsoo Kim. ]
> [ paulmck: Move slab definition per Stephen Rothwell and kbuild test robot. ]
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Introducing three functions, kmem_valid_obj(), kmem_provenance(),
mem_dump_obj() looks better than patchset v1. Nice work. Few comments
below.

> ---
>  include/linux/mm.h   |  2 ++
>  include/linux/slab.h |  2 ++
>  mm/slab.c            | 28 +++++++++++++++++++++
>  mm/slab.h            | 11 +++++++++
>  mm/slab_common.c     | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  mm/slob.c            |  7 ++++++
>  mm/slub.c            | 40 ++++++++++++++++++++++++++++++
>  mm/util.c            | 25 +++++++++++++++++++
>  8 files changed, 184 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ef360fe..1eea266 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3153,5 +3153,7 @@ unsigned long wp_shared_mapping_range(struct address_space *mapping,
>  
>  extern int sysctl_nr_trim_pages;
>  
> +void mem_dump_obj(void *object);
> +
>  #endif /* __KERNEL__ */
>  #endif /* _LINUX_MM_H */
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index dd6897f..169b511 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -186,6 +186,8 @@ void kfree(const void *);
>  void kfree_sensitive(const void *);
>  size_t __ksize(const void *);
>  size_t ksize(const void *);
> +bool kmem_valid_obj(void *object);
> +void kmem_dump_obj(void *object);
>  
>  #ifdef CONFIG_HAVE_HARDENED_USERCOPY_ALLOCATOR
>  void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
> diff --git a/mm/slab.c b/mm/slab.c
> index b111356..72b6743 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3602,6 +3602,34 @@ void *kmem_cache_alloc_node_trace(struct kmem_cache *cachep,
>  EXPORT_SYMBOL(kmem_cache_alloc_node_trace);
>  #endif
>  
> +void kmem_provenance(struct kmem_provenance *kpp)

To open up the possibility of future enhancement, name, provenance,
looks not good to me. This function could be used to extract various
object information so such as kmem_obj_info() looks better to me. Any
thought?

> +{
> +#ifdef DEBUG
> +	struct kmem_cache *cachep;
> +	void *object = kpp->kp_ptr;
> +	unsigned int objnr;
> +	void *objp;
> +	struct page *page = kpp->kp_page;
> +
> +	cachep = page->slab_cache;
> +	if (!(cachep->flags & SLAB_STORE_USER)) {
> +		kpp->kp_ret = NULL;
> +		goto nodebug;
> +	}
> +	objp = object - obj_offset(cachep);
> +	page = virt_to_head_page(objp);
> +	objnr = obj_to_index(cachep, page, objp);
> +	objp = index_to_obj(cachep, page, objnr);
> +	kpp->kp_objp = objp;
> +	kpp->kp_ret = *dbg_userword(cachep, objp);
> +nodebug:
> +#else
> +	kpp->kp_ret = NULL;
> +#endif
> +	if (kpp->kp_nstack)
> +		kpp->kp_stack[0] = NULL;
> +}
> +
>  static __always_inline void *
>  __do_kmalloc_node(size_t size, gfp_t flags, int node, unsigned long caller)
>  {
> diff --git a/mm/slab.h b/mm/slab.h
> index 6d7c6a5..28a41d5 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -630,4 +630,15 @@ static inline bool slab_want_init_on_free(struct kmem_cache *c)
>  	return false;
>  }
>  
> +#define KS_ADDRS_COUNT 16
> +struct kmem_provenance {
> +	void *kp_ptr;
> +	struct page *kp_page;
> +	void *kp_objp;
> +	void *kp_ret;
> +	void *kp_stack[KS_ADDRS_COUNT];
> +	int kp_nstack;
> +};
> +void kmem_provenance(struct kmem_provenance *kpp);
> +
>  #endif /* MM_SLAB_H */
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index f9ccd5d..09f0cbc 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -536,6 +536,75 @@ bool slab_is_available(void)
>  	return slab_state >= UP;
>  }
>  
> +/**
> + * kmem_valid_obj - does the pointer reference a valid slab object?
> + * @object: pointer to query.
> + *
> + * Return: %true if the pointer is to a not-yet-freed object from
> + * kmalloc() or kmem_cache_alloc(), either %true or %false if the pointer
> + * is to an already-freed object, and %false otherwise.
> + */
> +bool kmem_valid_obj(void *object)
> +{
> +	struct page *page;
> +
> +	if (!virt_addr_valid(object))
> +		return false;
> +	page = virt_to_head_page(object);
> +	return PageSlab(page);
> +}
> +EXPORT_SYMBOL_GPL(kmem_valid_obj);
> +
> +/**
> + * kmem_dump_obj - Print available slab provenance information
> + * @object: slab object for which to find provenance information.
> + *
> + * This function uses pr_cont(), so that the caller is expected to have
> + * printed out whatever preamble is appropriate.  The provenance information
> + * depends on the type of object and on how much debugging is enabled.
> + * For a slab-cache object, the fact that it is a slab object is printed,
> + * and, if available, the slab name, return address, and stack trace from
> + * the allocation of that object.
> + *
> + * This function will splat if passed a pointer to a non-slab object.
> + * If you are not sure what type of object you have, you should instead
> + * use mem_dump_obj().
> + */
> +void kmem_dump_obj(void *object)
> +{
> +	int i;
> +	struct page *page;
> +	struct kmem_provenance kp;
> +
> +	if (WARN_ON_ONCE(!virt_addr_valid(object)))
> +		return;
> +	page = virt_to_head_page(object);
> +	if (WARN_ON_ONCE(!PageSlab(page))) {
> +		pr_cont(" non-slab memory.\n");
> +		return;
> +	}
> +	kp.kp_ptr = object;
> +	kp.kp_page = page;
> +	kp.kp_nstack = KS_ADDRS_COUNT;

I hope that kmem_dump_obj() doesn't set any kp fields. It's the job
reserved for kmem_provenance().

> +	kmem_provenance(&kp);
> +	if (page->slab_cache)
> +		pr_cont(" slab %s", page->slab_cache->name);

Rather than accessing page->slab_cache, it's better to introduce
slab_cache field on kp and use it. Note that slob doesn't use
page->slab_cache. In slob, that field on struct page would be NULL so
it would not cause a problem. But using kp makes things clear.

> +	else
> +		pr_cont(" slab ");
> +	if (kp.kp_ret)
> +		pr_cont(" allocated at %pS\n", kp.kp_ret);
> +	else
> +		pr_cont("\n");
> +	if (kp.kp_stack[0]) {

This check would be useless since we check it on every iteration.
 
Thanks.

Re: [PATCH v2 sl-b 1/5] mm: Add mem_dump_obj() to print source of memory block

[Paul E. McKenney]

On Thu, Dec 10, 2020 at 09:04:11PM +0900, Joonsoo Kim wrote:
> On Tue, Dec 08, 2020 at 05:12:59PM -0800, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> > 
> > There are kernel facilities such as per-CPU reference counts that give
> > error messages in generic handlers or callbacks, whose messages are
> > unenlightening.  In the case of per-CPU reference-count underflow, this
> > is not a problem when creating a new use of this facility because in that
> > case the bug is almost certainly in the code implementing that new use.
> > However, trouble arises when deploying across many systems, which might
> > exercise corner cases that were not seen during development and testing.
> > Here, it would be really nice to get some kind of hint as to which of
> > several uses the underflow was caused by.
> > 
> > This commit therefore exposes a mem_dump_obj() function that takes
> > a pointer to memory (which must still be allocated if it has been
> > dynamically allocated) and prints available information on where that
> > memory came from.  This pointer can reference the middle of the block as
> > well as the beginning of the block, as needed by things like RCU callback
> > functions and timer handlers that might not know where the beginning of
> > the memory block is.  These functions and handlers can use mem_dump_obj()
> > to print out better hints as to where the problem might lie.
> > 
> > The information printed can depend on kernel configuration.  For example,
> > the allocation return address can be printed only for slab and slub,
> > and even then only when the necessary debug has been enabled.  For slab,
> > build with CONFIG_DEBUG_SLAB=y, and either use sizes with ample space
> > to the next power of two or use the SLAB_STORE_USER when creating the
> > kmem_cache structure.  For slub, build with CONFIG_SLUB_DEBUG=y and
> > boot with slub_debug=U, or pass SLAB_STORE_USER to kmem_cache_create()
> > if more focused use is desired.  Also for slub, use CONFIG_STACKTRACE
> > to enable printing of the allocation-time stack trace.
> > 
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Pekka Enberg <penberg@kernel.org>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: <linux-mm@kvack.org>
> > Reported-by: Andrii Nakryiko <andrii@kernel.org>
> > [ paulmck: Convert to printing and change names per Joonsoo Kim. ]
> > [ paulmck: Move slab definition per Stephen Rothwell and kbuild test robot. ]
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> Introducing three functions, kmem_valid_obj(), kmem_provenance(),
> mem_dump_obj() looks better than patchset v1. Nice work. Few comments
> below.

Glad you like it!

> > ---
> >  include/linux/mm.h   |  2 ++
> >  include/linux/slab.h |  2 ++
> >  mm/slab.c            | 28 +++++++++++++++++++++
> >  mm/slab.h            | 11 +++++++++
> >  mm/slab_common.c     | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  mm/slob.c            |  7 ++++++
> >  mm/slub.c            | 40 ++++++++++++++++++++++++++++++
> >  mm/util.c            | 25 +++++++++++++++++++
> >  8 files changed, 184 insertions(+)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index ef360fe..1eea266 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -3153,5 +3153,7 @@ unsigned long wp_shared_mapping_range(struct address_space *mapping,
> >  
> >  extern int sysctl_nr_trim_pages;
> >  
> > +void mem_dump_obj(void *object);
> > +
> >  #endif /* __KERNEL__ */
> >  #endif /* _LINUX_MM_H */
> > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > index dd6897f..169b511 100644
> > --- a/include/linux/slab.h
> > +++ b/include/linux/slab.h
> > @@ -186,6 +186,8 @@ void kfree(const void *);
> >  void kfree_sensitive(const void *);
> >  size_t __ksize(const void *);
> >  size_t ksize(const void *);
> > +bool kmem_valid_obj(void *object);
> > +void kmem_dump_obj(void *object);
> >  
> >  #ifdef CONFIG_HAVE_HARDENED_USERCOPY_ALLOCATOR
> >  void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
> > diff --git a/mm/slab.c b/mm/slab.c
> > index b111356..72b6743 100644
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -3602,6 +3602,34 @@ void *kmem_cache_alloc_node_trace(struct kmem_cache *cachep,
> >  EXPORT_SYMBOL(kmem_cache_alloc_node_trace);
> >  #endif
> >  
> > +void kmem_provenance(struct kmem_provenance *kpp)
> 
> To open up the possibility of future enhancement, name, provenance,
> looks not good to me. This function could be used to extract various
> object information so such as kmem_obj_info() looks better to me. Any
> thought?

The name kmem_obj_info() works for me, updated.

> > +{
> > +#ifdef DEBUG
> > +	struct kmem_cache *cachep;
> > +	void *object = kpp->kp_ptr;
> > +	unsigned int objnr;
> > +	void *objp;
> > +	struct page *page = kpp->kp_page;
> > +
> > +	cachep = page->slab_cache;
> > +	if (!(cachep->flags & SLAB_STORE_USER)) {
> > +		kpp->kp_ret = NULL;
> > +		goto nodebug;
> > +	}
> > +	objp = object - obj_offset(cachep);
> > +	page = virt_to_head_page(objp);
> > +	objnr = obj_to_index(cachep, page, objp);
> > +	objp = index_to_obj(cachep, page, objnr);
> > +	kpp->kp_objp = objp;
> > +	kpp->kp_ret = *dbg_userword(cachep, objp);
> > +nodebug:
> > +#else
> > +	kpp->kp_ret = NULL;
> > +#endif
> > +	if (kpp->kp_nstack)
> > +		kpp->kp_stack[0] = NULL;
> > +}
> > +
> >  static __always_inline void *
> >  __do_kmalloc_node(size_t size, gfp_t flags, int node, unsigned long caller)
> >  {
> > diff --git a/mm/slab.h b/mm/slab.h
> > index 6d7c6a5..28a41d5 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -630,4 +630,15 @@ static inline bool slab_want_init_on_free(struct kmem_cache *c)
> >  	return false;
> >  }
> >  
> > +#define KS_ADDRS_COUNT 16
> > +struct kmem_provenance {
> > +	void *kp_ptr;
> > +	struct page *kp_page;
> > +	void *kp_objp;
> > +	void *kp_ret;
> > +	void *kp_stack[KS_ADDRS_COUNT];
> > +	int kp_nstack;
> > +};
> > +void kmem_provenance(struct kmem_provenance *kpp);
> > +
> >  #endif /* MM_SLAB_H */
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index f9ccd5d..09f0cbc 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -536,6 +536,75 @@ bool slab_is_available(void)
> >  	return slab_state >= UP;
> >  }
> >  
> > +/**
> > + * kmem_valid_obj - does the pointer reference a valid slab object?
> > + * @object: pointer to query.
> > + *
> > + * Return: %true if the pointer is to a not-yet-freed object from
> > + * kmalloc() or kmem_cache_alloc(), either %true or %false if the pointer
> > + * is to an already-freed object, and %false otherwise.
> > + */
> > +bool kmem_valid_obj(void *object)
> > +{
> > +	struct page *page;
> > +
> > +	if (!virt_addr_valid(object))
> > +		return false;
> > +	page = virt_to_head_page(object);
> > +	return PageSlab(page);
> > +}
> > +EXPORT_SYMBOL_GPL(kmem_valid_obj);
> > +
> > +/**
> > + * kmem_dump_obj - Print available slab provenance information
> > + * @object: slab object for which to find provenance information.
> > + *
> > + * This function uses pr_cont(), so that the caller is expected to have
> > + * printed out whatever preamble is appropriate.  The provenance information
> > + * depends on the type of object and on how much debugging is enabled.
> > + * For a slab-cache object, the fact that it is a slab object is printed,
> > + * and, if available, the slab name, return address, and stack trace from
> > + * the allocation of that object.
> > + *
> > + * This function will splat if passed a pointer to a non-slab object.
> > + * If you are not sure what type of object you have, you should instead
> > + * use mem_dump_obj().
> > + */
> > +void kmem_dump_obj(void *object)
> > +{
> > +	int i;
> > +	struct page *page;
> > +	struct kmem_provenance kp;
> > +
> > +	if (WARN_ON_ONCE(!virt_addr_valid(object)))
> > +		return;
> > +	page = virt_to_head_page(object);
> > +	if (WARN_ON_ONCE(!PageSlab(page))) {
> > +		pr_cont(" non-slab memory.\n");
> > +		return;
> > +	}
> > +	kp.kp_ptr = object;
> > +	kp.kp_page = page;
> > +	kp.kp_nstack = KS_ADDRS_COUNT;
> 
> I hope that kmem_dump_obj() doesn't set any kp fields. It's the job
> reserved for kmem_provenance().

I assigned to kp.kp_ptr to avoid doing it in each of the three variants
of kmem_provenance(), but it is clearly not a big deal to do the three
assignments.  Ditto for kp.kp_page.

I can remove the kp.kp_nstack assignment entirely and have the variants
just use KS_ADDRS_COUNT directly.

And I will zero-initialize kp, thus getting rid of some of the
NULL/0 assignments in the various kmem_provenance() functions.
And a lot of goto statements.

> > +	kmem_provenance(&kp);
> > +	if (page->slab_cache)
> > +		pr_cont(" slab %s", page->slab_cache->name);
> 
> Rather than accessing page->slab_cache, it's better to introduce
> slab_cache field on kp and use it. Note that slob doesn't use
> page->slab_cache. In slob, that field on struct page would be NULL so
> it would not cause a problem. But using kp makes things clear.

Easy enough!

> > +	else
> > +		pr_cont(" slab ");
> > +	if (kp.kp_ret)
> > +		pr_cont(" allocated at %pS\n", kp.kp_ret);
> > +	else
> > +		pr_cont("\n");
> > +	if (kp.kp_stack[0]) {
> 
> This check would be useless since we check it on every iteration.

Good catch, removed.

							Thanx, Paul

[PATCH v2 sl-b 2/5] mm: Make mem_dump_obj() handle NULL and zero-sized pointers

[paulmck]

From: "Paul E. McKenney" <paulmck@kernel.org>

This commit makes mem_dump_obj() call out NULL and zero-sized pointers
specially instead of classifying them as non-paged memory.

Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <linux-mm@kvack.org>
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 mm/util.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index d0e60d2..8c2449f 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -985,7 +985,12 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
 void mem_dump_obj(void *object)
 {
 	if (!virt_addr_valid(object)) {
-		pr_cont(" non-paged (local) memory.\n");
+		if (object == NULL)
+			pr_cont(" NULL pointer.\n");
+		else if (object == ZERO_SIZE_PTR)
+			pr_cont(" zero-size pointer.\n");
+		else
+			pr_cont(" non-paged (local) memory.\n");
 		return;
 	}
 	if (kmem_valid_obj(object)) {
-- 
2.9.5

Re: [PATCH v2 sl-b 2/5] mm: Make mem_dump_obj() handle NULL and zero-sized pointers

[Vlastimil Babka]

On 12/9/20 2:13 AM, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> This commit makes mem_dump_obj() call out NULL and zero-sized pointers
> specially instead of classifying them as non-paged memory.
> 
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: <linux-mm@kvack.org>
> Reported-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  mm/util.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index d0e60d2..8c2449f 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -985,7 +985,12 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
>  void mem_dump_obj(void *object)
>  {
>  	if (!virt_addr_valid(object)) {
> -		pr_cont(" non-paged (local) memory.\n");
> +		if (object == NULL)
> +			pr_cont(" NULL pointer.\n");
> +		else if (object == ZERO_SIZE_PTR)
> +			pr_cont(" zero-size pointer.\n");
> +		else
> +			pr_cont(" non-paged (local) memory.\n");
>  		return;
>  	}
>  	if (kmem_valid_obj(object)) {
> 

Re: [PATCH v2 sl-b 2/5] mm: Make mem_dump_obj() handle NULL and zero-sized pointers

[Paul E. McKenney]

On Wed, Dec 09, 2020 at 06:48:47PM +0100, Vlastimil Babka wrote:
> On 12/9/20 2:13 AM, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> > 
> > This commit makes mem_dump_obj() call out NULL and zero-sized pointers
> > specially instead of classifying them as non-paged memory.
> > 
> > Cc: Christoph Lameter <cl@linux.com>
> > Cc: Pekka Enberg <penberg@kernel.org>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: <linux-mm@kvack.org>
> > Reported-by: Andrii Nakryiko <andrii@kernel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>

Applied, thank you!

						Thanx, Paul

> > ---
> >  mm/util.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/util.c b/mm/util.c
> > index d0e60d2..8c2449f 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -985,7 +985,12 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
> >  void mem_dump_obj(void *object)
> >  {
> >  	if (!virt_addr_valid(object)) {
> > -		pr_cont(" non-paged (local) memory.\n");
> > +		if (object == NULL)
> > +			pr_cont(" NULL pointer.\n");
> > +		else if (object == ZERO_SIZE_PTR)
> > +			pr_cont(" zero-size pointer.\n");
> > +		else
> > +			pr_cont(" non-paged (local) memory.\n");
> >  		return;
> >  	}
> >  	if (kmem_valid_obj(object)) {
> > 
> 

[PATCH v2 sl-b 3/5] mm: Make mem_dump_obj() handle vmalloc() memory

[paulmck]

From: "Paul E. McKenney" <paulmck@kernel.org>

This commit adds vmalloc() support to mem_dump_obj().  Note that the
vmalloc_dump_obj() function combines the checking and dumping, in
contrast with the split between kmem_valid_obj() and kmem_dump_obj().
The reason for the difference is that the checking in the vmalloc()
case involves acquiring a global lock, and redundant acquisitions of
global locks should be avoided, even on not-so-fast paths.

Note that this change causes on-stack variables to be reported as
vmalloc() storage from kernel_clone() or similar, depending on the degree
of inlining that your compiler does.  This is likely more helpful than
the earlier "non-paged (local) memory".

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: <linux-mm@kvack.org>
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 include/linux/vmalloc.h |  6 ++++++
 mm/util.c               | 12 +++++++-----
 mm/vmalloc.c            | 12 ++++++++++++
 3 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index 938eaf9..c89c2be 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -248,4 +248,10 @@ pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
 int register_vmap_purge_notifier(struct notifier_block *nb);
 int unregister_vmap_purge_notifier(struct notifier_block *nb);
 
+#ifdef CONFIG_MMU
+bool vmalloc_dump_obj(void *object);
+#else
+static inline bool vmalloc_dump_obj(void *object) { return false; }
+#endif
+
 #endif /* _LINUX_VMALLOC_H */
diff --git a/mm/util.c b/mm/util.c
index 8c2449f..ee99a0a 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -984,6 +984,12 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
  */
 void mem_dump_obj(void *object)
 {
+	if (kmem_valid_obj(object)) {
+		kmem_dump_obj(object);
+		return;
+	}
+	if (vmalloc_dump_obj(object))
+		return;
 	if (!virt_addr_valid(object)) {
 		if (object == NULL)
 			pr_cont(" NULL pointer.\n");
@@ -993,10 +999,6 @@ void mem_dump_obj(void *object)
 			pr_cont(" non-paged (local) memory.\n");
 		return;
 	}
-	if (kmem_valid_obj(object)) {
-		kmem_dump_obj(object);
-		return;
-	}
-	pr_cont(" non-slab memory.\n");
+	pr_cont(" non-slab/vmalloc memory.\n");
 }
 EXPORT_SYMBOL_GPL(mem_dump_obj);
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 6ae491a..7421719 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3431,6 +3431,18 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
 }
 #endif	/* CONFIG_SMP */
 
+bool vmalloc_dump_obj(void *object)
+{
+	struct vm_struct *vm;
+	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
+
+	vm = find_vm_area(objp);
+	if (!vm)
+		return false;
+	pr_cont(" vmalloc allocated at %pS\n", vm->caller);
+	return true;
+}
+
 #ifdef CONFIG_PROC_FS
 static void *s_start(struct seq_file *m, loff_t *pos)
 	__acquires(&vmap_purge_lock)
-- 
2.9.5

Re: [PATCH v2 sl-b 3/5] mm: Make mem_dump_obj() handle vmalloc() memory

[Vlastimil Babka]

On 12/9/20 2:13 AM, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> This commit adds vmalloc() support to mem_dump_obj().  Note that the
> vmalloc_dump_obj() function combines the checking and dumping, in
> contrast with the split between kmem_valid_obj() and kmem_dump_obj().
> The reason for the difference is that the checking in the vmalloc()
> case involves acquiring a global lock, and redundant acquisitions of
> global locks should be avoided, even on not-so-fast paths.
> 
> Note that this change causes on-stack variables to be reported as
> vmalloc() storage from kernel_clone() or similar, depending on the degree
> of inlining that your compiler does.  This is likely more helpful than
> the earlier "non-paged (local) memory".
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: <linux-mm@kvack.org>
> Reported-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

...

> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3431,6 +3431,18 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
>  }
>  #endif	/* CONFIG_SMP */
>  
> +bool vmalloc_dump_obj(void *object)
> +{
> +	struct vm_struct *vm;
> +	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> +
> +	vm = find_vm_area(objp);
> +	if (!vm)
> +		return false;
> +	pr_cont(" vmalloc allocated at %pS\n", vm->caller);

Would it be useful to print the vm area boundaries too?

> +	return true;
> +}
> +
>  #ifdef CONFIG_PROC_FS
>  static void *s_start(struct seq_file *m, loff_t *pos)
>  	__acquires(&vmap_purge_lock)
> 

Re: [PATCH v2 sl-b 3/5] mm: Make mem_dump_obj() handle vmalloc() memory

[Uladzislau Rezki]

On Wed, Dec 09, 2020 at 06:51:20PM +0100, Vlastimil Babka wrote:
> On 12/9/20 2:13 AM, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> > 
> > This commit adds vmalloc() support to mem_dump_obj().  Note that the
> > vmalloc_dump_obj() function combines the checking and dumping, in
> > contrast with the split between kmem_valid_obj() and kmem_dump_obj().
> > The reason for the difference is that the checking in the vmalloc()
> > case involves acquiring a global lock, and redundant acquisitions of
> > global locks should be avoided, even on not-so-fast paths.
> > 
> > Note that this change causes on-stack variables to be reported as
> > vmalloc() storage from kernel_clone() or similar, depending on the degree
> > of inlining that your compiler does.  This is likely more helpful than
> > the earlier "non-paged (local) memory".
> > 
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > Cc: <linux-mm@kvack.org>
> > Reported-by: Andrii Nakryiko <andrii@kernel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> ...
> 
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -3431,6 +3431,18 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
> >  }
> >  #endif	/* CONFIG_SMP */
> >  
> > +bool vmalloc_dump_obj(void *object)
> > +{
> > +	struct vm_struct *vm;
> > +	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> > +
> > +	vm = find_vm_area(objp);
> > +	if (!vm)
> > +		return false;
> > +	pr_cont(" vmalloc allocated at %pS\n", vm->caller);
> 
> Would it be useful to print the vm area boundaries too?
> 
Do you mean va_start/va_end information?

--
Vlad Rezki

Re: [PATCH v2 sl-b 3/5] mm: Make mem_dump_obj() handle vmalloc() memory

[Paul E. McKenney]

On Wed, Dec 09, 2020 at 06:51:20PM +0100, Vlastimil Babka wrote:
> On 12/9/20 2:13 AM, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> > 
> > This commit adds vmalloc() support to mem_dump_obj().  Note that the
> > vmalloc_dump_obj() function combines the checking and dumping, in
> > contrast with the split between kmem_valid_obj() and kmem_dump_obj().
> > The reason for the difference is that the checking in the vmalloc()
> > case involves acquiring a global lock, and redundant acquisitions of
> > global locks should be avoided, even on not-so-fast paths.
> > 
> > Note that this change causes on-stack variables to be reported as
> > vmalloc() storage from kernel_clone() or similar, depending on the degree
> > of inlining that your compiler does.  This is likely more helpful than
> > the earlier "non-paged (local) memory".
> > 
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > Cc: <linux-mm@kvack.org>
> > Reported-by: Andrii Nakryiko <andrii@kernel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> ...
> 
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -3431,6 +3431,18 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
> >  }
> >  #endif	/* CONFIG_SMP */
> >  
> > +bool vmalloc_dump_obj(void *object)
> > +{
> > +	struct vm_struct *vm;
> > +	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> > +
> > +	vm = find_vm_area(objp);
> > +	if (!vm)
> > +		return false;
> > +	pr_cont(" vmalloc allocated at %pS\n", vm->caller);
> 
> Would it be useful to print the vm area boundaries too?

Like this?

I also considered instead using vm->size, but that always seems to include
an extra page, so a 4-page span is listed as having 20480 bytes and a
one-page span is 8192 bytes.  This might be more accurate in some sense,
but would be quite confusing to someone trying to compare this size with
that requested in the vmalloc() call.

							Thanx, Paul

------------------------------------------------------------------------

commit 33e0469c289c2f78e5f0d0c463c8ee3357d273c0
Author: Paul E. McKenney <paulmck@kernel.org>
Date:   Wed Dec 9 15:15:27 2020 -0800

    mm: Make mem_obj_dump() vmalloc() dumps include start and length
    
    This commit adds the starting address and number of pages to the vmalloc()
    information dumped by way of vmalloc_dump_obj().
    
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: <linux-mm@kvack.org>
    Reported-by: Andrii Nakryiko <andrii@kernel.org>
    Suggested-by: Vlastimil Babka <vbabka@suse.cz>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 7421719..77b1100 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3439,7 +3439,8 @@ bool vmalloc_dump_obj(void *object)
 	vm = find_vm_area(objp);
 	if (!vm)
 		return false;
-	pr_cont(" vmalloc allocated at %pS\n", vm->caller);
+	pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
+		vm->nr_pages, (unsigned long)vm->addr, vm->caller);
 	return true;
 }
 

Re: [PATCH v2 sl-b 3/5] mm: Make mem_dump_obj() handle vmalloc() memory

[Vlastimil Babka]

On 12/10/20 12:23 AM, Paul E. McKenney wrote:
> On Wed, Dec 09, 2020 at 06:51:20PM +0100, Vlastimil Babka wrote:
>> On 12/9/20 2:13 AM, paulmck@kernel.org wrote:
>> > From: "Paul E. McKenney" <paulmck@kernel.org>
>> > 
>> > This commit adds vmalloc() support to mem_dump_obj().  Note that the
>> > vmalloc_dump_obj() function combines the checking and dumping, in
>> > contrast with the split between kmem_valid_obj() and kmem_dump_obj().
>> > The reason for the difference is that the checking in the vmalloc()
>> > case involves acquiring a global lock, and redundant acquisitions of
>> > global locks should be avoided, even on not-so-fast paths.
>> > 
>> > Note that this change causes on-stack variables to be reported as
>> > vmalloc() storage from kernel_clone() or similar, depending on the degree
>> > of inlining that your compiler does.  This is likely more helpful than
>> > the earlier "non-paged (local) memory".
>> > 
>> > Cc: Andrew Morton <akpm@linux-foundation.org>
>> > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>> > Cc: <linux-mm@kvack.org>
>> > Reported-by: Andrii Nakryiko <andrii@kernel.org>
>> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>> 
>> ...
>> 
>> > --- a/mm/vmalloc.c
>> > +++ b/mm/vmalloc.c
>> > @@ -3431,6 +3431,18 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
>> >  }
>> >  #endif	/* CONFIG_SMP */
>> >  
>> > +bool vmalloc_dump_obj(void *object)
>> > +{
>> > +	struct vm_struct *vm;
>> > +	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
>> > +
>> > +	vm = find_vm_area(objp);
>> > +	if (!vm)
>> > +		return false;
>> > +	pr_cont(" vmalloc allocated at %pS\n", vm->caller);
>> 
>> Would it be useful to print the vm area boundaries too?
> 
> Like this?

Yeah, thanks!

> I also considered instead using vm->size, but that always seems to include
> an extra page, so a 4-page span is listed as having 20480 bytes and a
> one-page span is 8192 bytes.  This might be more accurate in some sense,
> but would be quite confusing to someone trying to compare this size with
> that requested in the vmalloc() call.

Right.

> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 33e0469c289c2f78e5f0d0c463c8ee3357d273c0
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date:   Wed Dec 9 15:15:27 2020 -0800
> 
>     mm: Make mem_obj_dump() vmalloc() dumps include start and length
>     
>     This commit adds the starting address and number of pages to the vmalloc()
>     information dumped by way of vmalloc_dump_obj().
>     
>     Cc: Andrew Morton <akpm@linux-foundation.org>
>     Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>     Cc: <linux-mm@kvack.org>
>     Reported-by: Andrii Nakryiko <andrii@kernel.org>
>     Suggested-by: Vlastimil Babka <vbabka@suse.cz>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 7421719..77b1100 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3439,7 +3439,8 @@ bool vmalloc_dump_obj(void *object)
>  	vm = find_vm_area(objp);
>  	if (!vm)
>  		return false;
> -	pr_cont(" vmalloc allocated at %pS\n", vm->caller);
> +	pr_cont(" %u-page vmalloc region starting at %#lx allocated at %pS\n",
> +		vm->nr_pages, (unsigned long)vm->addr, vm->caller);
>  	return true;
>  }
>  
> 

Re: [PATCH v2 sl-b 3/5] mm: Make mem_dump_obj() handle vmalloc() memory

[Uladzislau Rezki]

On Tue, Dec 08, 2020 at 05:13:01PM -0800, paulmck@kernel.org wrote:
> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> This commit adds vmalloc() support to mem_dump_obj().  Note that the
> vmalloc_dump_obj() function combines the checking and dumping, in
> contrast with the split between kmem_valid_obj() and kmem_dump_obj().
> The reason for the difference is that the checking in the vmalloc()
> case involves acquiring a global lock, and redundant acquisitions of
> global locks should be avoided, even on not-so-fast paths.
> 
> Note that this change causes on-stack variables to be reported as
> vmalloc() storage from kernel_clone() or similar, depending on the degree
> of inlining that your compiler does.  This is likely more helpful than
> the earlier "non-paged (local) memory".
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: <linux-mm@kvack.org>
> Reported-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
>  include/linux/vmalloc.h |  6 ++++++
>  mm/util.c               | 12 +++++++-----
>  mm/vmalloc.c            | 12 ++++++++++++
>  3 files changed, 25 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 938eaf9..c89c2be 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -248,4 +248,10 @@ pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
>  int register_vmap_purge_notifier(struct notifier_block *nb);
>  int unregister_vmap_purge_notifier(struct notifier_block *nb);
>  
> +#ifdef CONFIG_MMU
> +bool vmalloc_dump_obj(void *object);
> +#else
> +static inline bool vmalloc_dump_obj(void *object) { return false; }
> +#endif
> +
>  #endif /* _LINUX_VMALLOC_H */
> diff --git a/mm/util.c b/mm/util.c
> index 8c2449f..ee99a0a 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -984,6 +984,12 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
>   */
>  void mem_dump_obj(void *object)
>  {
> +	if (kmem_valid_obj(object)) {
> +		kmem_dump_obj(object);
> +		return;
> +	}
> +	if (vmalloc_dump_obj(object))
> +		return;
>  	if (!virt_addr_valid(object)) {
>  		if (object == NULL)
>  			pr_cont(" NULL pointer.\n");
> @@ -993,10 +999,6 @@ void mem_dump_obj(void *object)
>  			pr_cont(" non-paged (local) memory.\n");
>  		return;
>  	}
> -	if (kmem_valid_obj(object)) {
> -		kmem_dump_obj(object);
> -		return;
> -	}
> -	pr_cont(" non-slab memory.\n");
> +	pr_cont(" non-slab/vmalloc memory.\n");
>  }
>  EXPORT_SYMBOL_GPL(mem_dump_obj);
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 6ae491a..7421719 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -3431,6 +3431,18 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
>  }
>  #endif	/* CONFIG_SMP */
>  
> +bool vmalloc_dump_obj(void *object)
> +{
> +	struct vm_struct *vm;
> +	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
>
Paul, vmalloced addresses are already aligned to PAGE_SIZE, so that one
is odd.

--
Vlad Rezki

Re: [PATCH v2 sl-b 3/5] mm: Make mem_dump_obj() handle vmalloc() memory

[Paul E. McKenney]

On Wed, Dec 09, 2020 at 08:36:37PM +0100, Uladzislau Rezki wrote:
> On Tue, Dec 08, 2020 at 05:13:01PM -0800, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.org>
> > 
> > This commit adds vmalloc() support to mem_dump_obj().  Note that the
> > vmalloc_dump_obj() function combines the checking and dumping, in
> > contrast with the split between kmem_valid_obj() and kmem_dump_obj().
> > The reason for the difference is that the checking in the vmalloc()
> > case involves acquiring a global lock, and redundant acquisitions of
> > global locks should be avoided, even on not-so-fast paths.
> > 
> > Note that this change causes on-stack variables to be reported as
> > vmalloc() storage from kernel_clone() or similar, depending on the degree
> > of inlining that your compiler does.  This is likely more helpful than
> > the earlier "non-paged (local) memory".
> > 
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > Cc: <linux-mm@kvack.org>
> > Reported-by: Andrii Nakryiko <andrii@kernel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> >  include/linux/vmalloc.h |  6 ++++++
> >  mm/util.c               | 12 +++++++-----
> >  mm/vmalloc.c            | 12 ++++++++++++
> >  3 files changed, 25 insertions(+), 5 deletions(-)
> > 
> > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > index 938eaf9..c89c2be 100644
> > --- a/include/linux/vmalloc.h
> > +++ b/include/linux/vmalloc.h
> > @@ -248,4 +248,10 @@ pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
> >  int register_vmap_purge_notifier(struct notifier_block *nb);
> >  int unregister_vmap_purge_notifier(struct notifier_block *nb);
> >  
> > +#ifdef CONFIG_MMU
> > +bool vmalloc_dump_obj(void *object);
> > +#else
> > +static inline bool vmalloc_dump_obj(void *object) { return false; }
> > +#endif
> > +
> >  #endif /* _LINUX_VMALLOC_H */
> > diff --git a/mm/util.c b/mm/util.c
> > index 8c2449f..ee99a0a 100644
> > --- a/mm/util.c
> > +++ b/mm/util.c
> > @@ -984,6 +984,12 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
> >   */
> >  void mem_dump_obj(void *object)
> >  {
> > +	if (kmem_valid_obj(object)) {
> > +		kmem_dump_obj(object);
> > +		return;
> > +	}
> > +	if (vmalloc_dump_obj(object))
> > +		return;
> >  	if (!virt_addr_valid(object)) {
> >  		if (object == NULL)
> >  			pr_cont(" NULL pointer.\n");
> > @@ -993,10 +999,6 @@ void mem_dump_obj(void *object)
> >  			pr_cont(" non-paged (local) memory.\n");
> >  		return;
> >  	}
> > -	if (kmem_valid_obj(object)) {
> > -		kmem_dump_obj(object);
> > -		return;
> > -	}
> > -	pr_cont(" non-slab memory.\n");
> > +	pr_cont(" non-slab/vmalloc memory.\n");
> >  }
> >  EXPORT_SYMBOL_GPL(mem_dump_obj);
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 6ae491a..7421719 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -3431,6 +3431,18 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
> >  }
> >  #endif	/* CONFIG_SMP */
> >  
> > +bool vmalloc_dump_obj(void *object)
> > +{
> > +	struct vm_struct *vm;
> > +	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> >
> Paul, vmalloced addresses are already aligned to PAGE_SIZE, so that one
> is odd.

They are, but this is to handle things like this:

	struct foo {
		int a;
		struct rcu_head rh;
	};

	void silly(struct foo *fp)
	{
		call_rcu(&fp->rh, my_rcu_cb);
		call_rcu(&fp->rh, my_other_rcu_cb);
	}

In kernels built with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y, this would
result in a call to mem_dump_obj() and then to vmalloc_dump_obj()
with a non-page-aligned pointer.

							Thanx, Paul

Re: [PATCH v2 sl-b 3/5] mm: Make mem_dump_obj() handle vmalloc() memory

[Uladzislau Rezki]

On Wed, Dec 09, 2020 at 11:42:39AM -0800, Paul E. McKenney wrote:
> On Wed, Dec 09, 2020 at 08:36:37PM +0100, Uladzislau Rezki wrote:
> > On Tue, Dec 08, 2020 at 05:13:01PM -0800, paulmck@kernel.org wrote:
> > > From: "Paul E. McKenney" <paulmck@kernel.org>
> > > 
> > > This commit adds vmalloc() support to mem_dump_obj().  Note that the
> > > vmalloc_dump_obj() function combines the checking and dumping, in
> > > contrast with the split between kmem_valid_obj() and kmem_dump_obj().
> > > The reason for the difference is that the checking in the vmalloc()
> > > case involves acquiring a global lock, and redundant acquisitions of
> > > global locks should be avoided, even on not-so-fast paths.
> > > 
> > > Note that this change causes on-stack variables to be reported as
> > > vmalloc() storage from kernel_clone() or similar, depending on the degree
> > > of inlining that your compiler does.  This is likely more helpful than
> > > the earlier "non-paged (local) memory".
> > > 
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > > Cc: <linux-mm@kvack.org>
> > > Reported-by: Andrii Nakryiko <andrii@kernel.org>
> > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > > ---
> > >  include/linux/vmalloc.h |  6 ++++++
> > >  mm/util.c               | 12 +++++++-----
> > >  mm/vmalloc.c            | 12 ++++++++++++
> > >  3 files changed, 25 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> > > index 938eaf9..c89c2be 100644
> > > --- a/include/linux/vmalloc.h
> > > +++ b/include/linux/vmalloc.h
> > > @@ -248,4 +248,10 @@ pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
> > >  int register_vmap_purge_notifier(struct notifier_block *nb);
> > >  int unregister_vmap_purge_notifier(struct notifier_block *nb);
> > >  
> > > +#ifdef CONFIG_MMU
> > > +bool vmalloc_dump_obj(void *object);
> > > +#else
> > > +static inline bool vmalloc_dump_obj(void *object) { return false; }
> > > +#endif
> > > +
> > >  #endif /* _LINUX_VMALLOC_H */
> > > diff --git a/mm/util.c b/mm/util.c
> > > index 8c2449f..ee99a0a 100644
> > > --- a/mm/util.c
> > > +++ b/mm/util.c
> > > @@ -984,6 +984,12 @@ int __weak memcmp_pages(struct page *page1, struct page *page2)
> > >   */
> > >  void mem_dump_obj(void *object)
> > >  {
> > > +	if (kmem_valid_obj(object)) {
> > > +		kmem_dump_obj(object);
> > > +		return;
> > > +	}
> > > +	if (vmalloc_dump_obj(object))
> > > +		return;
> > >  	if (!virt_addr_valid(object)) {
> > >  		if (object == NULL)
> > >  			pr_cont(" NULL pointer.\n");
> > > @@ -993,10 +999,6 @@ void mem_dump_obj(void *object)
> > >  			pr_cont(" non-paged (local) memory.\n");
> > >  		return;
> > >  	}
> > > -	if (kmem_valid_obj(object)) {
> > > -		kmem_dump_obj(object);
> > > -		return;
> > > -	}
> > > -	pr_cont(" non-slab memory.\n");
> > > +	pr_cont(" non-slab/vmalloc memory.\n");
> > >  }
> > >  EXPORT_SYMBOL_GPL(mem_dump_obj);
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index 6ae491a..7421719 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -3431,6 +3431,18 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms)
> > >  }
> > >  #endif	/* CONFIG_SMP */
> > >  
> > > +bool vmalloc_dump_obj(void *object)
> > > +{
> > > +	struct vm_struct *vm;
> > > +	void *objp = (void *)PAGE_ALIGN((unsigned long)object);
> > >
> > Paul, vmalloced addresses are already aligned to PAGE_SIZE, so that one
> > is odd.
> 
> They are, but this is to handle things like this:
> 
> 	struct foo {
> 		int a;
> 		struct rcu_head rh;
> 	};
> 
> 	void silly(struct foo *fp)
> 	{
> 		call_rcu(&fp->rh, my_rcu_cb);
> 		call_rcu(&fp->rh, my_other_rcu_cb);
> 	}
> 
> In kernels built with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y, this would
> result in a call to mem_dump_obj() and then to vmalloc_dump_obj()
> with a non-page-aligned pointer.
> 
OK, i got it. I thought the functions deals with original vmalloc
pointer. In fact it is not :)

--
Vlad Rezki

[PATCH v2 sl-b 4/5] rcu: Make call_rcu() print mem_dump_obj() info for double-freed callback

[paulmck]

From: "Paul E. McKenney" <paulmck@kernel.org>

The debug-object double-free checks in __call_rcu() print out the
RCU callback function, which is usually sufficient to track down the
double free.  However, all uses of things like queue_rcu_work() will
have the same RCU callback function (rcu_work_rcufn() in this case),
so a diagnostic message for a double queue_rcu_work() needs more than
just the callback function.

This commit therefore calls mem_dump_obj() to dump out any additional
available information on the double-freed callback.

Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <linux-mm@kvack.org>
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b6c9c49..464cf14 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2957,6 +2957,7 @@ static void check_cb_ovld(struct rcu_data *rdp)
 static void
 __call_rcu(struct rcu_head *head, rcu_callback_t func)
 {
+	static atomic_t doublefrees;
 	unsigned long flags;
 	struct rcu_data *rdp;
 	bool was_alldone;
@@ -2970,8 +2971,10 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
 		 * Use rcu:rcu_callback trace event to find the previous
 		 * time callback was passed to __call_rcu().
 		 */
-		WARN_ONCE(1, "__call_rcu(): Double-freed CB %p->%pS()!!!\n",
-			  head, head->func);
+		if (atomic_inc_return(&doublefrees) < 4) {
+			pr_err("%s(): Double-freed CB %p->%pS()!!!  ", __func__, head, head->func);
+			mem_dump_obj(head);
+		}
 		WRITE_ONCE(head->func, rcu_leak_callback);
 		return;
 	}
-- 
2.9.5