[PATCH v2 0/5] mm/slub: fix validation races and cleanup locking

[PATCH v2 0/5] mm/slub: fix validation races and cleanup locking

[Vlastimil Babka]

This series builds on the validation races fix posted previously [1]
that became patch 2 here and contains all the details in its
description.

Thanks to Hyeonggon Yoo's observation, patch 3 removes more slab_lock()
usage that became unnecessary after patch 2.

This made it possible to further simplify locking code in patches 4 and
5. Since those are related to PREEMPT_RT, I'm CCing relevant people on
this series.

Changes since v1 [2]:

- add acks/reviews from Hyeonggon and David
- minor fixes to patch 2 as reported by Hyeonggon
- patch 5 reworked to rely on disabled preemption by bit_spin_lock()
  which should be sufficient without disabled interrupts on RT

git version:

https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=slub-validate-fix-v2r2

I plan to add this series to slab.git for-next in few days.

[1] https://lore.kernel.org/all/20220809140043.9903-1-vbabka@suse.cz/
[2] https://lore.kernel.org/all/20220812091426.18418-1-vbabka@suse.cz/

Vlastimil Babka (5):
  mm/slub: move free_debug_processing() further
  mm/slub: restrict sysfs validation to debug caches and make it safe
  mm/slub: remove slab_lock() usage for debug operations
  mm/slub: convert object_map_lock to non-raw spinlock
  mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()

 mm/slub.c | 417 ++++++++++++++++++++++++++++++++----------------------
 1 file changed, 251 insertions(+), 166 deletions(-)

-- 
2.37.2

[PATCH v2 1/5] mm/slub: move free_debug_processing() further

[Vlastimil Babka]

In the following patch, the function free_debug_processing() will be
calling add_partial(), remove_partial() and discard_slab(), se move it
below their definitions to avoid forward declarations. To make review
easier, separate the move from functional changes.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
---
 mm/slub.c | 114 +++++++++++++++++++++++++++---------------------------
 1 file changed, 57 insertions(+), 57 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 862dbd9af4f5..87e794ab101a 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1385,63 +1385,6 @@ static inline int free_consistency_checks(struct kmem_cache *s,
 	return 1;
 }
 
-/* Supports checking bulk free of a constructed freelist */
-static noinline int free_debug_processing(
-	struct kmem_cache *s, struct slab *slab,
-	void *head, void *tail, int bulk_cnt,
-	unsigned long addr)
-{
-	struct kmem_cache_node *n = get_node(s, slab_nid(slab));
-	void *object = head;
-	int cnt = 0;
-	unsigned long flags, flags2;
-	int ret = 0;
-	depot_stack_handle_t handle = 0;
-
-	if (s->flags & SLAB_STORE_USER)
-		handle = set_track_prepare();
-
-	spin_lock_irqsave(&n->list_lock, flags);
-	slab_lock(slab, &flags2);
-
-	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
-		if (!check_slab(s, slab))
-			goto out;
-	}
-
-next_object:
-	cnt++;
-
-	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
-		if (!free_consistency_checks(s, slab, object, addr))
-			goto out;
-	}
-
-	if (s->flags & SLAB_STORE_USER)
-		set_track_update(s, object, TRACK_FREE, addr, handle);
-	trace(s, slab, object, 0);
-	/* Freepointer not overwritten by init_object(), SLAB_POISON moved it */
-	init_object(s, object, SLUB_RED_INACTIVE);
-
-	/* Reached end of constructed freelist yet? */
-	if (object != tail) {
-		object = get_freepointer(s, object);
-		goto next_object;
-	}
-	ret = 1;
-
-out:
-	if (cnt != bulk_cnt)
-		slab_err(s, slab, "Bulk freelist count(%d) invalid(%d)\n",
-			 bulk_cnt, cnt);
-
-	slab_unlock(slab, &flags2);
-	spin_unlock_irqrestore(&n->list_lock, flags);
-	if (!ret)
-		slab_fix(s, "Object at 0x%p not freed", object);
-	return ret;
-}
-
 /*
  * Parse a block of slub_debug options. Blocks are delimited by ';'
  *
@@ -2788,6 +2731,63 @@ static inline unsigned long node_nr_objs(struct kmem_cache_node *n)
 {
 	return atomic_long_read(&n->total_objects);
 }
+
+/* Supports checking bulk free of a constructed freelist */
+static noinline int free_debug_processing(
+	struct kmem_cache *s, struct slab *slab,
+	void *head, void *tail, int bulk_cnt,
+	unsigned long addr)
+{
+	struct kmem_cache_node *n = get_node(s, slab_nid(slab));
+	void *object = head;
+	int cnt = 0;
+	unsigned long flags, flags2;
+	int ret = 0;
+	depot_stack_handle_t handle = 0;
+
+	if (s->flags & SLAB_STORE_USER)
+		handle = set_track_prepare();
+
+	spin_lock_irqsave(&n->list_lock, flags);
+	slab_lock(slab, &flags2);
+
+	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
+		if (!check_slab(s, slab))
+			goto out;
+	}
+
+next_object:
+	cnt++;
+
+	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
+		if (!free_consistency_checks(s, slab, object, addr))
+			goto out;
+	}
+
+	if (s->flags & SLAB_STORE_USER)
+		set_track_update(s, object, TRACK_FREE, addr, handle);
+	trace(s, slab, object, 0);
+	/* Freepointer not overwritten by init_object(), SLAB_POISON moved it */
+	init_object(s, object, SLUB_RED_INACTIVE);
+
+	/* Reached end of constructed freelist yet? */
+	if (object != tail) {
+		object = get_freepointer(s, object);
+		goto next_object;
+	}
+	ret = 1;
+
+out:
+	if (cnt != bulk_cnt)
+		slab_err(s, slab, "Bulk freelist count(%d) invalid(%d)\n",
+			 bulk_cnt, cnt);
+
+	slab_unlock(slab, &flags2);
+	spin_unlock_irqrestore(&n->list_lock, flags);
+	if (!ret)
+		slab_fix(s, "Object at 0x%p not freed", object);
+	return ret;
+}
 #endif /* CONFIG_SLUB_DEBUG */
 
 #if defined(CONFIG_SLUB_DEBUG) || defined(CONFIG_SYSFS)
-- 
2.37.2

[PATCH v2 2/5] mm/slub: restrict sysfs validation to debug caches and make it safe

[Vlastimil Babka]

Rongwei Wang reports [1] that cache validation triggered by writing to
/sys/kernel/slab/<cache>/validate is racy against normal cache
operations (e.g. freeing) in a way that can cause false positive
inconsistency reports for caches with debugging enabled. The problem is
that debugging actions that mark object free or active and actual
freelist operations are not atomic, and the validation can see an
inconsistent state.

For caches that do or don't have debugging enabled, additional races
involving n->nr_slabs are possible that result in false reports of wrong
slab counts.

This patch attempts to solve these issues while not adding overhead to
normal (especially fastpath) operations for caches that do not have
debugging enabled. Such overhead would not be justified to make possible
userspace-triggered validation safe. Instead, disable the validation for
caches that don't have debugging enabled and make their sysfs validate
handler return -EINVAL.

For caches that do have debugging enabled, we can instead extend the
existing approach of not using percpu freelists to force all alloc/free
operations to the slow paths where debugging flags is checked and acted
upon. There can adjust the debug-specific paths to increase n->list_lock
coverage against concurrent validation as necessary.

The processing on free in free_debug_processing() already happens under
n->list_lock so we can extend it to actually do the freeing as well and
thus make it atomic against concurrent validation. As observed by
Hyeonggon Yoo, we do not really need to take slab_lock() anymore here
because all paths we could race with are protected by n->list_lock under
the new scheme, so drop its usage here.

The processing on alloc in alloc_debug_processing() currently doesn't
take any locks, but we have to first allocate the object from a slab on
the partial list (as debugging caches have no percpu slabs) and thus
take the n->list_lock anyway. Add a function alloc_single_from_partial()
that grabs just the allocated object instead of the whole freelist, and
does the debug processing. The n->list_lock coverage again makes it
atomic against validation and it is also ultimately more efficient than
the current grabbing of freelist immediately followed by slab
deactivation.

To prevent races on n->nr_slabs updates, make sure that for caches with
debugging enabled, inc_slabs_node() or dec_slabs_node() is called under
n->list_lock. When allocating a new slab for a debug cache, handle the
allocation by a new function alloc_single_from_new_slab() instead of the
current forced deactivation path.

Neither of these changes affect the fast paths at all. The changes in
slow paths are negligible for non-debug caches.

[1] https://lore.kernel.org/all/20220529081535.69275-1-rongwei.wang@linux.alibaba.com/

Reported-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 231 ++++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 179 insertions(+), 52 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 87e794ab101a..a5a913879871 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1324,17 +1324,14 @@ static inline int alloc_consistency_checks(struct kmem_cache *s,
 }
 
 static noinline int alloc_debug_processing(struct kmem_cache *s,
-					struct slab *slab,
-					void *object, unsigned long addr)
+					struct slab *slab, void *object)
 {
 	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
 		if (!alloc_consistency_checks(s, slab, object))
 			goto bad;
 	}
 
-	/* Success perform special debug activities for allocs */
-	if (s->flags & SLAB_STORE_USER)
-		set_track(s, object, TRACK_ALLOC, addr);
+	/* Success. Perform special debug activities for allocs */
 	trace(s, slab, object, 1);
 	init_object(s, object, SLUB_RED_ACTIVE);
 	return 1;
@@ -1604,16 +1601,18 @@ static inline
 void setup_slab_debug(struct kmem_cache *s, struct slab *slab, void *addr) {}
 
 static inline int alloc_debug_processing(struct kmem_cache *s,
-	struct slab *slab, void *object, unsigned long addr) { return 0; }
+	struct slab *slab, void *object) { return 0; }
 
-static inline int free_debug_processing(
+static inline void free_debug_processing(
 	struct kmem_cache *s, struct slab *slab,
 	void *head, void *tail, int bulk_cnt,
-	unsigned long addr) { return 0; }
+	unsigned long addr) {}
 
 static inline void slab_pad_check(struct kmem_cache *s, struct slab *slab) {}
 static inline int check_object(struct kmem_cache *s, struct slab *slab,
 			void *object, u8 val) { return 1; }
+static inline void set_track(struct kmem_cache *s, void *object,
+			     enum track_item alloc, unsigned long addr) {}
 static inline void add_full(struct kmem_cache *s, struct kmem_cache_node *n,
 					struct slab *slab) {}
 static inline void remove_full(struct kmem_cache *s, struct kmem_cache_node *n,
@@ -1919,11 +1918,13 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
 		 */
 		slab = alloc_slab_page(alloc_gfp, node, oo);
 		if (unlikely(!slab))
-			goto out;
+			return NULL;
 		stat(s, ORDER_FALLBACK);
 	}
 
 	slab->objects = oo_objects(oo);
+	slab->inuse = 0;
+	slab->frozen = 0;
 
 	account_slab(slab, oo_order(oo), s, flags);
 
@@ -1950,15 +1951,6 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
 		set_freepointer(s, p, NULL);
 	}
 
-	slab->inuse = slab->objects;
-	slab->frozen = 1;
-
-out:
-	if (!slab)
-		return NULL;
-
-	inc_slabs_node(s, slab_nid(slab), slab->objects);
-
 	return slab;
 }
 
@@ -2045,6 +2037,75 @@ static inline void remove_partial(struct kmem_cache_node *n,
 	n->nr_partial--;
 }
 
+/*
+ * Called only for kmem_cache_debug() caches instead of acquire_slab(), with a
+ * slab from the n->partial list. Remove only a single object from the slab, do
+ * the alloc_debug_processing() checks and leave the slab on the list, or move
+ * it to full list if it was the last free object.
+ */
+static void *alloc_single_from_partial(struct kmem_cache *s,
+		struct kmem_cache_node *n, struct slab *slab)
+{
+	void *object;
+
+	lockdep_assert_held(&n->list_lock);
+
+	object = slab->freelist;
+	slab->freelist = get_freepointer(s, object);
+	slab->inuse++;
+
+	if (!alloc_debug_processing(s, slab, object)) {
+		remove_partial(n, slab);
+		return NULL;
+	}
+
+	if (slab->inuse == slab->objects) {
+		remove_partial(n, slab);
+		add_full(s, n, slab);
+	}
+
+	return object;
+}
+
+/*
+ * Called only for kmem_cache_debug() caches to allocate from a freshly
+ * allocated slab. Allocate a single object instead of whole freelist
+ * and put the slab to the partial (or full) list.
+ */
+static void *alloc_single_from_new_slab(struct kmem_cache *s,
+					struct slab *slab)
+{
+	int nid = slab_nid(slab);
+	struct kmem_cache_node *n = get_node(s, nid);
+	unsigned long flags;
+	void *object;
+
+
+	object = slab->freelist;
+	slab->freelist = get_freepointer(s, object);
+	slab->inuse = 1;
+
+	if (!alloc_debug_processing(s, slab, object))
+		/*
+		 * It's not really expected that this would fail on a
+		 * freshly allocated slab, but a concurrent memory
+		 * corruption in theory could cause that.
+		 */
+		return NULL;
+
+	spin_lock_irqsave(&n->list_lock, flags);
+
+	if (slab->inuse == slab->objects)
+		add_full(s, n, slab);
+	else
+		add_partial(n, slab, DEACTIVATE_TO_HEAD);
+
+	inc_slabs_node(s, nid, slab->objects);
+	spin_unlock_irqrestore(&n->list_lock, flags);
+
+	return object;
+}
+
 /*
  * Remove slab from the partial list, freeze it and
  * return the pointer to the freelist.
@@ -2125,6 +2186,13 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
 		if (!pfmemalloc_match(slab, gfpflags))
 			continue;
 
+		if (kmem_cache_debug(s)) {
+			object = alloc_single_from_partial(s, n, slab);
+			if (object)
+				break;
+			continue;
+		}
+
 		t = acquire_slab(s, n, slab, object == NULL);
 		if (!t)
 			break;
@@ -2733,31 +2801,39 @@ static inline unsigned long node_nr_objs(struct kmem_cache_node *n)
 }
 
 /* Supports checking bulk free of a constructed freelist */
-static noinline int free_debug_processing(
+static noinline void free_debug_processing(
 	struct kmem_cache *s, struct slab *slab,
 	void *head, void *tail, int bulk_cnt,
 	unsigned long addr)
 {
 	struct kmem_cache_node *n = get_node(s, slab_nid(slab));
+	struct slab *slab_free = NULL;
 	void *object = head;
 	int cnt = 0;
-	unsigned long flags, flags2;
-	int ret = 0;
+	unsigned long flags;
+	bool checks_ok = false;
 	depot_stack_handle_t handle = 0;
 
 	if (s->flags & SLAB_STORE_USER)
 		handle = set_track_prepare();
 
 	spin_lock_irqsave(&n->list_lock, flags);
-	slab_lock(slab, &flags2);
 
 	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
 		if (!check_slab(s, slab))
 			goto out;
 	}
 
+	if (slab->inuse < bulk_cnt) {
+		slab_err(s, slab, "Slab has %d allocated objects but %d are to be freed\n",
+			 slab->inuse, bulk_cnt);
+		goto out;
+	}
+
 next_object:
-	cnt++;
+
+	if (++cnt > bulk_cnt)
+		goto out_cnt;
 
 	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
 		if (!free_consistency_checks(s, slab, object, addr))
@@ -2775,18 +2851,56 @@ static noinline int free_debug_processing(
 		object = get_freepointer(s, object);
 		goto next_object;
 	}
-	ret = 1;
+	checks_ok = true;
 
-out:
+out_cnt:
 	if (cnt != bulk_cnt)
-		slab_err(s, slab, "Bulk freelist count(%d) invalid(%d)\n",
+		slab_err(s, slab, "Bulk free expected %d objects but found %d\n",
 			 bulk_cnt, cnt);
 
-	slab_unlock(slab, &flags2);
+out:
+	if (checks_ok) {
+		void *prior = slab->freelist;
+
+		/* Perform the actual freeing while we still hold the locks */
+		slab->inuse -= cnt;
+		set_freepointer(s, tail, prior);
+		slab->freelist = head;
+
+		/* Do we need to remove the slab from full or partial list? */
+		if (!prior) {
+			remove_full(s, n, slab);
+		} else if (slab->inuse == 0) {
+			remove_partial(n, slab);
+			stat(s, FREE_REMOVE_PARTIAL);
+		}
+
+		/* Do we need to discard the slab or add to partial list? */
+		if (slab->inuse == 0) {
+			slab_free = slab;
+		} else if (!prior) {
+			add_partial(n, slab, DEACTIVATE_TO_TAIL);
+			stat(s, FREE_ADD_PARTIAL);
+		}
+	}
+
+	if (slab_free) {
+		/*
+		 * Update the counters while still holding n->list_lock to
+		 * prevent spurious validation warnings
+		 */
+		dec_slabs_node(s, slab_nid(slab_free), slab_free->objects);
+	}
+
 	spin_unlock_irqrestore(&n->list_lock, flags);
-	if (!ret)
+
+	if (!checks_ok)
 		slab_fix(s, "Object at 0x%p not freed", object);
-	return ret;
+
+	if (slab_free) {
+		stat(s, FREE_SLAB);
+		free_slab(s, slab_free);
+	}
 }
 #endif /* CONFIG_SLUB_DEBUG */
 
@@ -3036,36 +3150,52 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 		return NULL;
 	}
 
+	stat(s, ALLOC_SLAB);
+
+	if (kmem_cache_debug(s)) {
+		freelist = alloc_single_from_new_slab(s, slab);
+
+		if (unlikely(!freelist))
+			goto new_objects;
+
+		if (s->flags & SLAB_STORE_USER)
+			set_track(s, freelist, TRACK_ALLOC, addr);
+
+		return freelist;
+	}
+
 	/*
 	 * No other reference to the slab yet so we can
 	 * muck around with it freely without cmpxchg
 	 */
 	freelist = slab->freelist;
 	slab->freelist = NULL;
+	slab->inuse = slab->objects;
+	slab->frozen = 1;
 
-	stat(s, ALLOC_SLAB);
+	inc_slabs_node(s, slab_nid(slab), slab->objects);
 
 check_new_slab:
 
 	if (kmem_cache_debug(s)) {
-		if (!alloc_debug_processing(s, slab, freelist, addr)) {
-			/* Slab failed checks. Next slab needed */
-			goto new_slab;
-		} else {
-			/*
-			 * For debug case, we don't load freelist so that all
-			 * allocations go through alloc_debug_processing()
-			 */
-			goto return_single;
-		}
+		/*
+		 * For debug caches here we had to go through
+		 * alloc_single_from_partial() so just store the tracking info
+		 * and return the object
+		 */
+		if (s->flags & SLAB_STORE_USER)
+			set_track(s, freelist, TRACK_ALLOC, addr);
+		return freelist;
 	}
 
-	if (unlikely(!pfmemalloc_match(slab, gfpflags)))
+	if (unlikely(!pfmemalloc_match(slab, gfpflags))) {
 		/*
 		 * For !pfmemalloc_match() case we don't load freelist so that
 		 * we don't make further mismatched allocations easier.
 		 */
-		goto return_single;
+		deactivate_slab(s, slab, get_freepointer(s, freelist));
+		return freelist;
+	}
 
 retry_load_slab:
 
@@ -3089,11 +3219,6 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 	c->slab = slab;
 
 	goto load_freelist;
-
-return_single:
-
-	deactivate_slab(s, slab, get_freepointer(s, freelist));
-	return freelist;
 }
 
 /*
@@ -3341,9 +3466,10 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
 	if (kfence_free(head))
 		return;
 
-	if (kmem_cache_debug(s) &&
-	    !free_debug_processing(s, slab, head, tail, cnt, addr))
+	if (kmem_cache_debug(s)) {
+		free_debug_processing(s, slab, head, tail, cnt, addr);
 		return;
+	}
 
 	do {
 		if (unlikely(n)) {
@@ -3936,6 +4062,7 @@ static void early_kmem_cache_node_alloc(int node)
 	slab = new_slab(kmem_cache_node, GFP_NOWAIT, node);
 
 	BUG_ON(!slab);
+	inc_slabs_node(kmem_cache_node, slab_nid(slab), slab->objects);
 	if (slab_nid(slab) != node) {
 		pr_err("SLUB: Unable to allocate memory from node %d\n", node);
 		pr_err("SLUB: Allocating a useless per node structure in order to be able to continue\n");
@@ -3950,7 +4077,6 @@ static void early_kmem_cache_node_alloc(int node)
 	n = kasan_slab_alloc(kmem_cache_node, n, GFP_KERNEL, false);
 	slab->freelist = get_freepointer(kmem_cache_node, n);
 	slab->inuse = 1;
-	slab->frozen = 0;
 	kmem_cache_node->node[node] = n;
 	init_kmem_cache_node(n);
 	inc_slabs_node(kmem_cache_node, node, slab->objects);
@@ -4611,6 +4737,7 @@ static int __kmem_cache_do_shrink(struct kmem_cache *s)
 			if (free == slab->objects) {
 				list_move(&slab->slab_list, &discard);
 				n->nr_partial--;
+				dec_slabs_node(s, node, slab->objects);
 			} else if (free <= SHRINK_PROMOTE_MAX)
 				list_move(&slab->slab_list, promote + free - 1);
 		}
@@ -4626,7 +4753,7 @@ static int __kmem_cache_do_shrink(struct kmem_cache *s)
 
 		/* Release empty slabs */
 		list_for_each_entry_safe(slab, t, &discard, slab_list)
-			discard_slab(s, slab);
+			free_slab(s, slab);
 
 		if (slabs_node(s, node))
 			ret = 1;
@@ -5601,7 +5728,7 @@ static ssize_t validate_store(struct kmem_cache *s,
 {
 	int ret = -EINVAL;
 
-	if (buf[0] == '1') {
+	if (buf[0] == '1' && kmem_cache_debug(s)) {
 		ret = validate_slab_cache(s);
 		if (ret >= 0)
 			ret = length;
-- 
2.37.2

Re: [PATCH v2 2/5] mm/slub: restrict sysfs validation to debug caches and make it safe

[Hyeonggon Yoo]

On Tue, Aug 23, 2022 at 07:03:57PM +0200, Vlastimil Babka wrote:
> Rongwei Wang reports [1] that cache validation triggered by writing to
> /sys/kernel/slab/<cache>/validate is racy against normal cache
> operations (e.g. freeing) in a way that can cause false positive
> inconsistency reports for caches with debugging enabled. The problem is
> that debugging actions that mark object free or active and actual
> freelist operations are not atomic, and the validation can see an
> inconsistent state.
> 
> For caches that do or don't have debugging enabled, additional races
> involving n->nr_slabs are possible that result in false reports of wrong
> slab counts.
> 
> This patch attempts to solve these issues while not adding overhead to
> normal (especially fastpath) operations for caches that do not have
> debugging enabled. Such overhead would not be justified to make possible
> userspace-triggered validation safe. Instead, disable the validation for
> caches that don't have debugging enabled and make their sysfs validate
> handler return -EINVAL.
> 
> For caches that do have debugging enabled, we can instead extend the
> existing approach of not using percpu freelists to force all alloc/free
> operations to the slow paths where debugging flags is checked and acted
> upon. There can adjust the debug-specific paths to increase n->list_lock
> coverage against concurrent validation as necessary.
> 
> The processing on free in free_debug_processing() already happens under
> n->list_lock so we can extend it to actually do the freeing as well and
> thus make it atomic against concurrent validation. As observed by
> Hyeonggon Yoo, we do not really need to take slab_lock() anymore here
> because all paths we could race with are protected by n->list_lock under
> the new scheme, so drop its usage here.
> 
> The processing on alloc in alloc_debug_processing() currently doesn't
> take any locks, but we have to first allocate the object from a slab on
> the partial list (as debugging caches have no percpu slabs) and thus
> take the n->list_lock anyway. Add a function alloc_single_from_partial()
> that grabs just the allocated object instead of the whole freelist, and
> does the debug processing. The n->list_lock coverage again makes it
> atomic against validation and it is also ultimately more efficient than
> the current grabbing of freelist immediately followed by slab
> deactivation.
> 
> To prevent races on n->nr_slabs updates, make sure that for caches with
> debugging enabled, inc_slabs_node() or dec_slabs_node() is called under
> n->list_lock. When allocating a new slab for a debug cache, handle the
> allocation by a new function alloc_single_from_new_slab() instead of the
> current forced deactivation path.
> 
> Neither of these changes affect the fast paths at all. The changes in
> slow paths are negligible for non-debug caches.
> 
> [1] https://lore.kernel.org/all/20220529081535.69275-1-rongwei.wang@linux.alibaba.com/
> 
> Reported-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/slub.c | 231 ++++++++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 179 insertions(+), 52 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 87e794ab101a..a5a913879871 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1324,17 +1324,14 @@ static inline int alloc_consistency_checks(struct kmem_cache *s,
>  }
>  
>  static noinline int alloc_debug_processing(struct kmem_cache *s,
> -					struct slab *slab,
> -					void *object, unsigned long addr)
> +					struct slab *slab, void *object)
>  {
>  	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
>  		if (!alloc_consistency_checks(s, slab, object))
>  			goto bad;
>  	}
>  
> -	/* Success perform special debug activities for allocs */
> -	if (s->flags & SLAB_STORE_USER)
> -		set_track(s, object, TRACK_ALLOC, addr);
> +	/* Success. Perform special debug activities for allocs */
>  	trace(s, slab, object, 1);
>  	init_object(s, object, SLUB_RED_ACTIVE);
>  	return 1;
> @@ -1604,16 +1601,18 @@ static inline
>  void setup_slab_debug(struct kmem_cache *s, struct slab *slab, void *addr) {}
>  
>  static inline int alloc_debug_processing(struct kmem_cache *s,
> -	struct slab *slab, void *object, unsigned long addr) { return 0; }
> +	struct slab *slab, void *object) { return 0; }
>  
> -static inline int free_debug_processing(
> +static inline void free_debug_processing(
>  	struct kmem_cache *s, struct slab *slab,
>  	void *head, void *tail, int bulk_cnt,
> -	unsigned long addr) { return 0; }
> +	unsigned long addr) {}
>  
>  static inline void slab_pad_check(struct kmem_cache *s, struct slab *slab) {}
>  static inline int check_object(struct kmem_cache *s, struct slab *slab,
>  			void *object, u8 val) { return 1; }
> +static inline void set_track(struct kmem_cache *s, void *object,
> +			     enum track_item alloc, unsigned long addr) {}
>  static inline void add_full(struct kmem_cache *s, struct kmem_cache_node *n,
>  					struct slab *slab) {}
>  static inline void remove_full(struct kmem_cache *s, struct kmem_cache_node *n,
> @@ -1919,11 +1918,13 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>  		 */
>  		slab = alloc_slab_page(alloc_gfp, node, oo);
>  		if (unlikely(!slab))
> -			goto out;
> +			return NULL;
>  		stat(s, ORDER_FALLBACK);
>  	}
>  
>  	slab->objects = oo_objects(oo);
> +	slab->inuse = 0;
> +	slab->frozen = 0;
>  
>  	account_slab(slab, oo_order(oo), s, flags);
>  
> @@ -1950,15 +1951,6 @@ static struct slab *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>  		set_freepointer(s, p, NULL);
>  	}
>  
> -	slab->inuse = slab->objects;
> -	slab->frozen = 1;
> -
> -out:
> -	if (!slab)
> -		return NULL;
> -
> -	inc_slabs_node(s, slab_nid(slab), slab->objects);
> -
>  	return slab;
>  }
>  
> @@ -2045,6 +2037,75 @@ static inline void remove_partial(struct kmem_cache_node *n,
>  	n->nr_partial--;
>  }
>  
> +/*
> + * Called only for kmem_cache_debug() caches instead of acquire_slab(), with a
> + * slab from the n->partial list. Remove only a single object from the slab, do
> + * the alloc_debug_processing() checks and leave the slab on the list, or move
> + * it to full list if it was the last free object.
> + */
> +static void *alloc_single_from_partial(struct kmem_cache *s,
> +		struct kmem_cache_node *n, struct slab *slab)
> +{
> +	void *object;
> +
> +	lockdep_assert_held(&n->list_lock);
> +
> +	object = slab->freelist;
> +	slab->freelist = get_freepointer(s, object);
> +	slab->inuse++;
> +
> +	if (!alloc_debug_processing(s, slab, object)) {
> +		remove_partial(n, slab);
> +		return NULL;
> +	}
> +
> +	if (slab->inuse == slab->objects) {
> +		remove_partial(n, slab);
> +		add_full(s, n, slab);
> +	}
> +
> +	return object;
> +}
> +
> +/*
> + * Called only for kmem_cache_debug() caches to allocate from a freshly
> + * allocated slab. Allocate a single object instead of whole freelist
> + * and put the slab to the partial (or full) list.
> + */
> +static void *alloc_single_from_new_slab(struct kmem_cache *s,
> +					struct slab *slab)
> +{
> +	int nid = slab_nid(slab);
> +	struct kmem_cache_node *n = get_node(s, nid);
> +	unsigned long flags;
> +	void *object;
> +
> +
> +	object = slab->freelist;
> +	slab->freelist = get_freepointer(s, object);
> +	slab->inuse = 1;
> +
> +	if (!alloc_debug_processing(s, slab, object))
> +		/*
> +		 * It's not really expected that this would fail on a
> +		 * freshly allocated slab, but a concurrent memory
> +		 * corruption in theory could cause that.
> +		 */
> +		return NULL;
> +
> +	spin_lock_irqsave(&n->list_lock, flags);
> +
> +	if (slab->inuse == slab->objects)
> +		add_full(s, n, slab);
> +	else
> +		add_partial(n, slab, DEACTIVATE_TO_HEAD);
> +
> +	inc_slabs_node(s, nid, slab->objects);
> +	spin_unlock_irqrestore(&n->list_lock, flags);
> +
> +	return object;
> +}
> +
>  /*
>   * Remove slab from the partial list, freeze it and
>   * return the pointer to the freelist.
> @@ -2125,6 +2186,13 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
>  		if (!pfmemalloc_match(slab, gfpflags))
>  			continue;
>  
> +		if (kmem_cache_debug(s)) {
> +			object = alloc_single_from_partial(s, n, slab);
> +			if (object)
> +				break;
> +			continue;
> +		}
> +
>  		t = acquire_slab(s, n, slab, object == NULL);
>  		if (!t)
>  			break;
> @@ -2733,31 +2801,39 @@ static inline unsigned long node_nr_objs(struct kmem_cache_node *n)
>  }
>  
>  /* Supports checking bulk free of a constructed freelist */
> -static noinline int free_debug_processing(
> +static noinline void free_debug_processing(
>  	struct kmem_cache *s, struct slab *slab,
>  	void *head, void *tail, int bulk_cnt,
>  	unsigned long addr)
>  {
>  	struct kmem_cache_node *n = get_node(s, slab_nid(slab));
> +	struct slab *slab_free = NULL;
>  	void *object = head;
>  	int cnt = 0;
> -	unsigned long flags, flags2;
> -	int ret = 0;
> +	unsigned long flags;
> +	bool checks_ok = false;
>  	depot_stack_handle_t handle = 0;
>  
>  	if (s->flags & SLAB_STORE_USER)
>  		handle = set_track_prepare();
>  
>  	spin_lock_irqsave(&n->list_lock, flags);
> -	slab_lock(slab, &flags2);
>  
>  	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
>  		if (!check_slab(s, slab))
>  			goto out;
>  	}
>  
> +	if (slab->inuse < bulk_cnt) {
> +		slab_err(s, slab, "Slab has %d allocated objects but %d are to be freed\n",
> +			 slab->inuse, bulk_cnt);
> +		goto out;
> +	}
> +
>  next_object:
> -	cnt++;
> +
> +	if (++cnt > bulk_cnt)
> +		goto out_cnt;
>  
>  	if (s->flags & SLAB_CONSISTENCY_CHECKS) {
>  		if (!free_consistency_checks(s, slab, object, addr))
> @@ -2775,18 +2851,56 @@ static noinline int free_debug_processing(
>  		object = get_freepointer(s, object);
>  		goto next_object;
>  	}
> -	ret = 1;
> +	checks_ok = true;
>  
> -out:
> +out_cnt:
>  	if (cnt != bulk_cnt)
> -		slab_err(s, slab, "Bulk freelist count(%d) invalid(%d)\n",
> +		slab_err(s, slab, "Bulk free expected %d objects but found %d\n",
>  			 bulk_cnt, cnt);
>  
> -	slab_unlock(slab, &flags2);
> +out:
> +	if (checks_ok) {
> +		void *prior = slab->freelist;
> +
> +		/* Perform the actual freeing while we still hold the locks */
> +		slab->inuse -= cnt;
> +		set_freepointer(s, tail, prior);
> +		slab->freelist = head;
> +
> +		/* Do we need to remove the slab from full or partial list? */
> +		if (!prior) {
> +			remove_full(s, n, slab);
> +		} else if (slab->inuse == 0) {
> +			remove_partial(n, slab);
> +			stat(s, FREE_REMOVE_PARTIAL);
> +		}
> +
> +		/* Do we need to discard the slab or add to partial list? */
> +		if (slab->inuse == 0) {
> +			slab_free = slab;
> +		} else if (!prior) {
> +			add_partial(n, slab, DEACTIVATE_TO_TAIL);
> +			stat(s, FREE_ADD_PARTIAL);
> +		}
> +	}
> +
> +	if (slab_free) {
> +		/*
> +		 * Update the counters while still holding n->list_lock to
> +		 * prevent spurious validation warnings
> +		 */
> +		dec_slabs_node(s, slab_nid(slab_free), slab_free->objects);
> +	}
> +
>  	spin_unlock_irqrestore(&n->list_lock, flags);
> -	if (!ret)
> +
> +	if (!checks_ok)
>  		slab_fix(s, "Object at 0x%p not freed", object);
> -	return ret;
> +
> +	if (slab_free) {
> +		stat(s, FREE_SLAB);
> +		free_slab(s, slab_free);
> +	}
>  }
>  #endif /* CONFIG_SLUB_DEBUG */
>  
> @@ -3036,36 +3150,52 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>  		return NULL;
>  	}
>  
> +	stat(s, ALLOC_SLAB);
> +
> +	if (kmem_cache_debug(s)) {
> +		freelist = alloc_single_from_new_slab(s, slab);
> +
> +		if (unlikely(!freelist))
> +			goto new_objects;
> +
> +		if (s->flags & SLAB_STORE_USER)
> +			set_track(s, freelist, TRACK_ALLOC, addr);
> +
> +		return freelist;
> +	}
> +
>  	/*
>  	 * No other reference to the slab yet so we can
>  	 * muck around with it freely without cmpxchg
>  	 */
>  	freelist = slab->freelist;
>  	slab->freelist = NULL;
> +	slab->inuse = slab->objects;
> +	slab->frozen = 1;
>  
> -	stat(s, ALLOC_SLAB);
> +	inc_slabs_node(s, slab_nid(slab), slab->objects);
>  
>  check_new_slab:
>  
>  	if (kmem_cache_debug(s)) {
> -		if (!alloc_debug_processing(s, slab, freelist, addr)) {
> -			/* Slab failed checks. Next slab needed */
> -			goto new_slab;
> -		} else {
> -			/*
> -			 * For debug case, we don't load freelist so that all
> -			 * allocations go through alloc_debug_processing()
> -			 */
> -			goto return_single;
> -		}
> +		/*
> +		 * For debug caches here we had to go through
> +		 * alloc_single_from_partial() so just store the tracking info
> +		 * and return the object
> +		 */
> +		if (s->flags & SLAB_STORE_USER)
> +			set_track(s, freelist, TRACK_ALLOC, addr);
> +		return freelist;
>  	}
>  
> -	if (unlikely(!pfmemalloc_match(slab, gfpflags)))
> +	if (unlikely(!pfmemalloc_match(slab, gfpflags))) {
>  		/*
>  		 * For !pfmemalloc_match() case we don't load freelist so that
>  		 * we don't make further mismatched allocations easier.
>  		 */
> -		goto return_single;
> +		deactivate_slab(s, slab, get_freepointer(s, freelist));
> +		return freelist;
> +	}
>  
>  retry_load_slab:
>  
> @@ -3089,11 +3219,6 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>  	c->slab = slab;
>  
>  	goto load_freelist;
> -
> -return_single:
> -
> -	deactivate_slab(s, slab, get_freepointer(s, freelist));
> -	return freelist;
>  }
>  
>  /*
> @@ -3341,9 +3466,10 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>  	if (kfence_free(head))
>  		return;
>  
> -	if (kmem_cache_debug(s) &&
> -	    !free_debug_processing(s, slab, head, tail, cnt, addr))
> +	if (kmem_cache_debug(s)) {
> +		free_debug_processing(s, slab, head, tail, cnt, addr);
>  		return;
> +	}
>  
>  	do {
>  		if (unlikely(n)) {
> @@ -3936,6 +4062,7 @@ static void early_kmem_cache_node_alloc(int node)
>  	slab = new_slab(kmem_cache_node, GFP_NOWAIT, node);
>  
>  	BUG_ON(!slab);
> +	inc_slabs_node(kmem_cache_node, slab_nid(slab), slab->objects);
>  	if (slab_nid(slab) != node) {
>  		pr_err("SLUB: Unable to allocate memory from node %d\n", node);
>  		pr_err("SLUB: Allocating a useless per node structure in order to be able to continue\n");
> @@ -3950,7 +4077,6 @@ static void early_kmem_cache_node_alloc(int node)
>  	n = kasan_slab_alloc(kmem_cache_node, n, GFP_KERNEL, false);
>  	slab->freelist = get_freepointer(kmem_cache_node, n);
>  	slab->inuse = 1;
> -	slab->frozen = 0;
>  	kmem_cache_node->node[node] = n;
>  	init_kmem_cache_node(n);
>  	inc_slabs_node(kmem_cache_node, node, slab->objects);
> @@ -4611,6 +4737,7 @@ static int __kmem_cache_do_shrink(struct kmem_cache *s)
>  			if (free == slab->objects) {
>  				list_move(&slab->slab_list, &discard);
>  				n->nr_partial--;
> +				dec_slabs_node(s, node, slab->objects);
>  			} else if (free <= SHRINK_PROMOTE_MAX)
>  				list_move(&slab->slab_list, promote + free - 1);
>  		}
> @@ -4626,7 +4753,7 @@ static int __kmem_cache_do_shrink(struct kmem_cache *s)
>  
>  		/* Release empty slabs */
>  		list_for_each_entry_safe(slab, t, &discard, slab_list)
> -			discard_slab(s, slab);
> +			free_slab(s, slab);
>  
>  		if (slabs_node(s, node))
>  			ret = 1;
> @@ -5601,7 +5728,7 @@ static ssize_t validate_store(struct kmem_cache *s,
>  {
>  	int ret = -EINVAL;
>  
> -	if (buf[0] == '1') {
> +	if (buf[0] == '1' && kmem_cache_debug(s)) {
>  		ret = validate_slab_cache(s);
>  		if (ret >= 0)
>  			ret = length;
> -- 
> 2.37.2

Looks good to me.

Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

-- 
Thanks,
Hyeonggon

[PATCH v2 3/5] mm/slub: remove slab_lock() usage for debug operations

[Vlastimil Babka]

All alloc and free operations on debug caches are now serialized by
n->list_lock, so we can remove slab_lock() usage in validate_slab()
and list_slab_objects() as those also happen under n->list_lock.

Note the usage in list_slab_objects() could happen even on non-debug
caches, but only during cache shutdown time, so there should not be any
parallel freeing activity anymore. Except for buggy slab users, but in
that case the slab_lock() would not help against the common cmpxchg
based fast paths (in non-debug caches) anyway.

Also adjust documentation comments accordingly.

Suggested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
---
 mm/slub.c | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index a5a913879871..b4065e892f7c 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -50,7 +50,7 @@
  *   1. slab_mutex (Global Mutex)
  *   2. node->list_lock (Spinlock)
  *   3. kmem_cache->cpu_slab->lock (Local lock)
- *   4. slab_lock(slab) (Only on some arches or for debugging)
+ *   4. slab_lock(slab) (Only on some arches)
  *   5. object_map_lock (Only for debugging)
  *
  *   slab_mutex
@@ -64,8 +64,9 @@
  *   The slab_lock is a wrapper around the page lock, thus it is a bit
  *   spinlock.
  *
- *   The slab_lock is only used for debugging and on arches that do not
- *   have the ability to do a cmpxchg_double. It only protects:
+ *   The slab_lock is only used on arches that do not have the ability
+ *   to do a cmpxchg_double. It only protects:
+ *
  *	A. slab->freelist	-> List of free objects in a slab
  *	B. slab->inuse		-> Number of objects in use
  *	C. slab->objects	-> Number of objects in slab
@@ -94,6 +95,9 @@
  *   allocating a long series of objects that fill up slabs does not require
  *   the list lock.
  *
+ *   For debug caches, all allocations are forced to go through a list_lock
+ *   protected region to serialize against concurrent validation.
+ *
  *   cpu_slab->lock local lock
  *
  *   This locks protect slowpath manipulation of all kmem_cache_cpu fields
@@ -4368,7 +4372,6 @@ static void list_slab_objects(struct kmem_cache *s, struct slab *slab,
 	void *p;
 
 	slab_err(s, slab, text, s->name);
-	slab_lock(slab, &flags);
 
 	map = get_map(s, slab);
 	for_each_object(p, s, addr, slab->objects) {
@@ -4379,7 +4382,6 @@ static void list_slab_objects(struct kmem_cache *s, struct slab *slab,
 		}
 	}
 	put_map(map);
-	slab_unlock(slab, &flags);
 #endif
 }
 
@@ -5107,12 +5109,9 @@ static void validate_slab(struct kmem_cache *s, struct slab *slab,
 {
 	void *p;
 	void *addr = slab_address(slab);
-	unsigned long flags;
-
-	slab_lock(slab, &flags);
 
 	if (!check_slab(s, slab) || !on_freelist(s, slab, NULL))
-		goto unlock;
+		return;
 
 	/* Now we know that a valid freelist exists */
 	__fill_map(obj_map, s, slab);
@@ -5123,8 +5122,6 @@ static void validate_slab(struct kmem_cache *s, struct slab *slab,
 		if (!check_object(s, slab, p, val))
 			break;
 	}
-unlock:
-	slab_unlock(slab, &flags);
 }
 
 static int validate_slab_node(struct kmem_cache *s,
-- 
2.37.2

[PATCH v2 4/5] mm/slub: convert object_map_lock to non-raw spinlock

[Vlastimil Babka]

The only remaining user of object_map_lock is list_slab_objects().
Obtaining the lock there used to happen under slab_lock() which implied
disabling irqs on PREEMPT_RT, thus it's a raw_spinlock. With the
slab_lock() removed, we can convert it to a normal spinlock.

Also remove the get_map()/put_map() wrappers as list_slab_objects()
became their only remaining user.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
---
 mm/slub.c | 36 ++++++------------------------------
 1 file changed, 6 insertions(+), 30 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index b4065e892f7c..0444a2ba4f12 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -565,7 +565,7 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
 
 #ifdef CONFIG_SLUB_DEBUG
 static unsigned long object_map[BITS_TO_LONGS(MAX_OBJS_PER_PAGE)];
-static DEFINE_RAW_SPINLOCK(object_map_lock);
+static DEFINE_SPINLOCK(object_map_lock);
 
 static void __fill_map(unsigned long *obj_map, struct kmem_cache *s,
 		       struct slab *slab)
@@ -599,30 +599,6 @@ static bool slab_add_kunit_errors(void)
 static inline bool slab_add_kunit_errors(void) { return false; }
 #endif
 
-/*
- * Determine a map of objects in use in a slab.
- *
- * Node listlock must be held to guarantee that the slab does
- * not vanish from under us.
- */
-static unsigned long *get_map(struct kmem_cache *s, struct slab *slab)
-	__acquires(&object_map_lock)
-{
-	VM_BUG_ON(!irqs_disabled());
-
-	raw_spin_lock(&object_map_lock);
-
-	__fill_map(object_map, s, slab);
-
-	return object_map;
-}
-
-static void put_map(unsigned long *map) __releases(&object_map_lock)
-{
-	VM_BUG_ON(map != object_map);
-	raw_spin_unlock(&object_map_lock);
-}
-
 static inline unsigned int size_from_object(struct kmem_cache *s)
 {
 	if (s->flags & SLAB_RED_ZONE)
@@ -4367,21 +4343,21 @@ static void list_slab_objects(struct kmem_cache *s, struct slab *slab,
 {
 #ifdef CONFIG_SLUB_DEBUG
 	void *addr = slab_address(slab);
-	unsigned long flags;
-	unsigned long *map;
 	void *p;
 
 	slab_err(s, slab, text, s->name);
 
-	map = get_map(s, slab);
+	spin_lock(&object_map_lock);
+	__fill_map(object_map, s, slab);
+
 	for_each_object(p, s, addr, slab->objects) {
 
-		if (!test_bit(__obj_to_index(s, addr, p), map)) {
+		if (!test_bit(__obj_to_index(s, addr, p), object_map)) {
 			pr_err("Object 0x%p @offset=%tu\n", p, p - addr);
 			print_tracking(s, p);
 		}
 	}
-	put_map(map);
+	spin_unlock(&object_map_lock);
 #endif
 }
 
-- 
2.37.2

Re: [PATCH v2 4/5] mm/slub: convert object_map_lock to non-raw spinlock

[Sebastian Andrzej Siewior]

On 2022-08-23 19:03:59 [+0200], Vlastimil Babka wrote:
> The only remaining user of object_map_lock is list_slab_objects().
> Obtaining the lock there used to happen under slab_lock() which implied
> disabling irqs on PREEMPT_RT, thus it's a raw_spinlock. With the
> slab_lock() removed, we can convert it to a normal spinlock.
> 
> Also remove the get_map()/put_map() wrappers as list_slab_objects()
> became their only remaining user.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: David Rientjes <rientjes@google.com>
> Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Sebastian

[PATCH v2 5/5] mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()

[Vlastimil Babka]

The PREEMPT_RT specific disabling of irqs in __cmpxchg_double_slab()
(through slab_[un]lock()) is unnecessary as bit_spin_lock() disables
preemption and that's sufficient on RT where interrupts are threaded.

That means we no longer need the slab_[un]lock() wrappers, so delete
them and rename the current __slab_[un]lock() to slab_[un]lock().

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
---
 mm/slub.c | 39 ++++++++++++---------------------------
 1 file changed, 12 insertions(+), 27 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 0444a2ba4f12..bb8c1292d7e8 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -446,7 +446,7 @@ slub_set_cpu_partial(struct kmem_cache *s, unsigned int nr_objects)
 /*
  * Per slab locking using the pagelock
  */
-static __always_inline void __slab_lock(struct slab *slab)
+static __always_inline void slab_lock(struct slab *slab)
 {
 	struct page *page = slab_page(slab);
 
@@ -454,7 +454,7 @@ static __always_inline void __slab_lock(struct slab *slab)
 	bit_spin_lock(PG_locked, &page->flags);
 }
 
-static __always_inline void __slab_unlock(struct slab *slab)
+static __always_inline void slab_unlock(struct slab *slab)
 {
 	struct page *page = slab_page(slab);
 
@@ -462,24 +462,12 @@ static __always_inline void __slab_unlock(struct slab *slab)
 	__bit_spin_unlock(PG_locked, &page->flags);
 }
 
-static __always_inline void slab_lock(struct slab *slab, unsigned long *flags)
-{
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		local_irq_save(*flags);
-	__slab_lock(slab);
-}
-
-static __always_inline void slab_unlock(struct slab *slab, unsigned long *flags)
-{
-	__slab_unlock(slab);
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		local_irq_restore(*flags);
-}
-
 /*
  * Interrupts must be disabled (for the fallback code to work right), typically
- * by an _irqsave() lock variant. Except on PREEMPT_RT where locks are different
- * so we disable interrupts as part of slab_[un]lock().
+ * by an _irqsave() lock variant. Except on PREEMPT_RT where these variants do
+ * not actually disable interrupts. On the other hand the migrate_disable()
+ * done by bit_spin_lock() is sufficient on PREEMPT_RT thanks to its threaded
+ * interrupts.
  */
 static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
 		void *freelist_old, unsigned long counters_old,
@@ -498,18 +486,15 @@ static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab
 	} else
 #endif
 	{
-		/* init to 0 to prevent spurious warnings */
-		unsigned long flags = 0;
-
-		slab_lock(slab, &flags);
+		slab_lock(slab);
 		if (slab->freelist == freelist_old &&
 					slab->counters == counters_old) {
 			slab->freelist = freelist_new;
 			slab->counters = counters_new;
-			slab_unlock(slab, &flags);
+			slab_unlock(slab);
 			return true;
 		}
-		slab_unlock(slab, &flags);
+		slab_unlock(slab);
 	}
 
 	cpu_relax();
@@ -540,16 +525,16 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
 		unsigned long flags;
 
 		local_irq_save(flags);
-		__slab_lock(slab);
+		slab_lock(slab);
 		if (slab->freelist == freelist_old &&
 					slab->counters == counters_old) {
 			slab->freelist = freelist_new;
 			slab->counters = counters_new;
-			__slab_unlock(slab);
+			slab_unlock(slab);
 			local_irq_restore(flags);
 			return true;
 		}
-		__slab_unlock(slab);
+		slab_unlock(slab);
 		local_irq_restore(flags);
 	}
 
-- 
2.37.2

Re: [PATCH v2 5/5] mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()

[Hyeonggon Yoo]

On Tue, Aug 23, 2022 at 07:04:00PM +0200, Vlastimil Babka wrote:
> The PREEMPT_RT specific disabling of irqs in __cmpxchg_double_slab()
> (through slab_[un]lock()) is unnecessary as bit_spin_lock() disables
> preemption and that's sufficient on RT where interrupts are threaded.
> 
> That means we no longer need the slab_[un]lock() wrappers, so delete
> them and rename the current __slab_[un]lock() to slab_[un]lock().
>

I'm not familiar with PREEMPT_RT preemption model so not sure I'm following.

1) Does "interrupts are threaded on RT" mean processing _most_ (all handlers
   that did not specified IRQF_NO_THREAD) of interrupts are delayed to irq threads
   and processed later in process context, and the kernel *never* use
   spinlock_t, local_lock_t that does not disable interrupts (and sleep) on RT
   in hardware/software interrupt context?

2) Do we need disabling irq in cmpxchg_double_slab() on RT?

BTW Is there a good documentation/papers on PREEMPT_RT preemption model?
I tried to find but only found Documentation/locking/locktypes.rst :(

Thanks!

> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: David Rientjes <rientjes@google.com>
> ---
>  mm/slub.c | 39 ++++++++++++---------------------------
>  1 file changed, 12 insertions(+), 27 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 0444a2ba4f12..bb8c1292d7e8 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -446,7 +446,7 @@ slub_set_cpu_partial(struct kmem_cache *s, unsigned int nr_objects)
>  /*
>   * Per slab locking using the pagelock
>   */
> -static __always_inline void __slab_lock(struct slab *slab)
> +static __always_inline void slab_lock(struct slab *slab)
>  {
>  	struct page *page = slab_page(slab);
>  
> @@ -454,7 +454,7 @@ static __always_inline void __slab_lock(struct slab *slab)
>  	bit_spin_lock(PG_locked, &page->flags);
>  }
>  
> -static __always_inline void __slab_unlock(struct slab *slab)
> +static __always_inline void slab_unlock(struct slab *slab)
>  {
>  	struct page *page = slab_page(slab);
>  
> @@ -462,24 +462,12 @@ static __always_inline void __slab_unlock(struct slab *slab)
>  	__bit_spin_unlock(PG_locked, &page->flags);
>  }
>  
> -static __always_inline void slab_lock(struct slab *slab, unsigned long *flags)
> -{
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		local_irq_save(*flags);
> -	__slab_lock(slab);
> -}
> -
> -static __always_inline void slab_unlock(struct slab *slab, unsigned long *flags)
> -{
> -	__slab_unlock(slab);
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		local_irq_restore(*flags);
> -}
> -
>  /*
>   * Interrupts must be disabled (for the fallback code to work right), typically
> - * by an _irqsave() lock variant. Except on PREEMPT_RT where locks are different
> - * so we disable interrupts as part of slab_[un]lock().
> + * by an _irqsave() lock variant. Except on PREEMPT_RT where these variants do
> + * not actually disable interrupts. On the other hand the migrate_disable()
> + * done by bit_spin_lock() is sufficient on PREEMPT_RT thanks to its threaded
> + * interrupts.
>   */
>  static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
>  		void *freelist_old, unsigned long counters_old,
> @@ -498,18 +486,15 @@ static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab
>  	} else
>  #endif
>  	{
> -		/* init to 0 to prevent spurious warnings */
> -		unsigned long flags = 0;
> -
> -		slab_lock(slab, &flags);
> +		slab_lock(slab);
>  		if (slab->freelist == freelist_old &&
>  					slab->counters == counters_old) {
>  			slab->freelist = freelist_new;
>  			slab->counters = counters_new;
> -			slab_unlock(slab, &flags);
> +			slab_unlock(slab);
>  			return true;
>  		}
> -		slab_unlock(slab, &flags);
> +		slab_unlock(slab);
>  	}
>  
>  	cpu_relax();
> @@ -540,16 +525,16 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
>  		unsigned long flags;
>  
>  		local_irq_save(flags);
> -		__slab_lock(slab);
> +		slab_lock(slab);
>  		if (slab->freelist == freelist_old &&
>  					slab->counters == counters_old) {
>  			slab->freelist = freelist_new;
>  			slab->counters = counters_new;
> -			__slab_unlock(slab);
> +			slab_unlock(slab);
>  			local_irq_restore(flags);
>  			return true;
>  		}
> -		__slab_unlock(slab);
> +		slab_unlock(slab);
>  		local_irq_restore(flags);
>  	}
>  
> -- 
> 2.37.2
> 

-- 
Thanks,
Hyeonggon

Re: [PATCH v2 5/5] mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()

[Vlastimil Babka]

On 8/24/22 12:24, Hyeonggon Yoo wrote:
> On Tue, Aug 23, 2022 at 07:04:00PM +0200, Vlastimil Babka wrote:
>> The PREEMPT_RT specific disabling of irqs in __cmpxchg_double_slab()
>> (through slab_[un]lock()) is unnecessary as bit_spin_lock() disables
>> preemption and that's sufficient on RT where interrupts are threaded.
>>
>> That means we no longer need the slab_[un]lock() wrappers, so delete
>> them and rename the current __slab_[un]lock() to slab_[un]lock().
>>
> 
> I'm not familiar with PREEMPT_RT preemption model so not sure I'm following.
> 
> 1) Does "interrupts are threaded on RT" mean processing _most_ (all handlers
>     that did not specified IRQF_NO_THREAD) of interrupts are delayed to irq threads
>     and processed later in process context, and the kernel *never* use
>     spinlock_t, local_lock_t that does not disable interrupts (and sleep) on RT
>     in hardware/software interrupt context?

AFAIK, yes, that's the case. So if some non-threaded handler used slab, 
we would be in trouble. But that would already be the case before this 
patch due to the local_lock usage in other paths - the bit_spin_lock() 
without disabled irq shouldn't add anything new here AFAIK.

> 2) Do we need disabling irq in cmpxchg_double_slab() on RT?

By that logic, we don't. But IMHO it's not worth complicating the code 
by special casing it for some negligible performance gain (the protected 
sections are very short), like we now special case 
__cmpxchg_double_slab() for correctness (after this patch, just the 
correctness of lockdep_assert_irqs_disabled()).

> BTW Is there a good documentation/papers on PREEMPT_RT preemption model?
> I tried to find but only found Documentation/locking/locktypes.rst :(

Good question, I don't know myself, maybe the RT guys do.

> Thanks!
> 

Re: [PATCH v2 5/5] mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()

[Hyeonggon Yoo]

On Wed, Aug 24, 2022 at 01:51:24PM +0200, Vlastimil Babka wrote:
> On 8/24/22 12:24, Hyeonggon Yoo wrote:
> > On Tue, Aug 23, 2022 at 07:04:00PM +0200, Vlastimil Babka wrote:
> > > The PREEMPT_RT specific disabling of irqs in __cmpxchg_double_slab()
> > > (through slab_[un]lock()) is unnecessary as bit_spin_lock() disables
> > > preemption and that's sufficient on RT where interrupts are threaded.
> > > 
> > > That means we no longer need the slab_[un]lock() wrappers, so delete
> > > them and rename the current __slab_[un]lock() to slab_[un]lock().
> > > 
> > 
> > I'm not familiar with PREEMPT_RT preemption model so not sure I'm following.
> > 
> > 1) Does "interrupts are threaded on RT" mean processing _most_ (all handlers
> >     that did not specified IRQF_NO_THREAD) of interrupts are delayed to irq threads
> >     and processed later in process context, and the kernel *never* use
> >     spinlock_t, local_lock_t that does not disable interrupts (and sleep) on RT
> >     in hardware/software interrupt context?
> 
> AFAIK, yes, that's the case. So if some non-threaded handler used slab, we
> would be in trouble.

Yeah, that was exactly what I wondered!

> But that would already be the case before this patch
> due to the local_lock usage in other paths - the bit_spin_lock() without
> disabled irq shouldn't add anything new here AFAIK.

Agreed.
 
> > 2) Do we need disabling irq in cmpxchg_double_slab() on RT?
> 
> By that logic, we don't. But IMHO it's not worth complicating the code by
> special casing it for some negligible performance gain (the protected
> sections are very short), like we now special case __cmpxchg_double_slab()
> for correctness (after this patch, just the correctness of
> lockdep_assert_irqs_disabled()).

Okay. Wanted to make sure that disabling interrupts is not required
by RT.

> 
> > BTW Is there a good documentation/papers on PREEMPT_RT preemption model?
> > I tried to find but only found Documentation/locking/locktypes.rst :(
> 
> Good question, I don't know myself, maybe the RT guys do.

Okay.

Thanks!

-- 
Thanks,
Hyeonggon

Re: [PATCH v2 5/5] mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()

[Sebastian Andrzej Siewior]

On 2022-08-24 19:24:20 [+0900], Hyeonggon Yoo wrote:
> I'm not familiar with PREEMPT_RT preemption model so not sure I'm following.
> 
> 1) Does "interrupts are threaded on RT" mean processing _most_ (all handlers
>    that did not specified IRQF_NO_THREAD) of interrupts are delayed to irq threads
>    and processed later in process context, and the kernel *never* use
>    spinlock_t, local_lock_t that does not disable interrupts (and sleep) on RT
>    in hardware/software interrupt context?

All non-threaded interrupts (or everything in hardirq context) must not
allocate (even with GFP_ATOMIC) or free memory on PREEMPT_RT. This is
policy. If you refer by "software interrupt" to softirqs then they can
allocate memory since softirq is also threaded.

> BTW Is there a good documentation/papers on PREEMPT_RT preemption model?
> I tried to find but only found Documentation/locking/locktypes.rst :(

What is it, that you are looking for? But the locking description is
good ;)

> Thanks!
> 
> > Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> > Acked-by: David Rientjes <rientjes@google.com>

Sebastian

Re: [PATCH v2 5/5] mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()

[Hyeonggon Yoo]

On Tue, Aug 23, 2022 at 07:04:00PM +0200, Vlastimil Babka wrote:
> The PREEMPT_RT specific disabling of irqs in __cmpxchg_double_slab()
> (through slab_[un]lock()) is unnecessary as bit_spin_lock() disables
> preemption and that's sufficient on RT where interrupts are threaded.
> 
> That means we no longer need the slab_[un]lock() wrappers, so delete
> them and rename the current __slab_[un]lock() to slab_[un]lock().
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: David Rientjes <rientjes@google.com>
> ---
>  mm/slub.c | 39 ++++++++++++---------------------------
>  1 file changed, 12 insertions(+), 27 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 0444a2ba4f12..bb8c1292d7e8 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -446,7 +446,7 @@ slub_set_cpu_partial(struct kmem_cache *s, unsigned int nr_objects)
>  /*
>   * Per slab locking using the pagelock
>   */
> -static __always_inline void __slab_lock(struct slab *slab)
> +static __always_inline void slab_lock(struct slab *slab)
>  {
>  	struct page *page = slab_page(slab);
>  
> @@ -454,7 +454,7 @@ static __always_inline void __slab_lock(struct slab *slab)
>  	bit_spin_lock(PG_locked, &page->flags);
>  }
>  
> -static __always_inline void __slab_unlock(struct slab *slab)
> +static __always_inline void slab_unlock(struct slab *slab)
>  {
>  	struct page *page = slab_page(slab);
>  
> @@ -462,24 +462,12 @@ static __always_inline void __slab_unlock(struct slab *slab)
>  	__bit_spin_unlock(PG_locked, &page->flags);
>  }
>  
> -static __always_inline void slab_lock(struct slab *slab, unsigned long *flags)
> -{
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		local_irq_save(*flags);
> -	__slab_lock(slab);
> -}
> -
> -static __always_inline void slab_unlock(struct slab *slab, unsigned long *flags)
> -{
> -	__slab_unlock(slab);
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT))
> -		local_irq_restore(*flags);
> -}
> -
>  /*
>   * Interrupts must be disabled (for the fallback code to work right), typically
> - * by an _irqsave() lock variant. Except on PREEMPT_RT where locks are different
> - * so we disable interrupts as part of slab_[un]lock().
> + * by an _irqsave() lock variant. Except on PREEMPT_RT where these variants do
> + * not actually disable interrupts. On the other hand the migrate_disable()

You mean preempt_disable()?
migrate_disable() will not be enough.

> + * done by bit_spin_lock() is sufficient on PREEMPT_RT thanks to its threaded
> + * interrupts.
>   */
>  static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
>  		void *freelist_old, unsigned long counters_old,
> @@ -498,18 +486,15 @@ static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab
>  	} else
>  #endif
>  	{
> -		/* init to 0 to prevent spurious warnings */
> -		unsigned long flags = 0;
> -
> -		slab_lock(slab, &flags);
> +		slab_lock(slab);
>  		if (slab->freelist == freelist_old &&
>  					slab->counters == counters_old) {
>  			slab->freelist = freelist_new;
>  			slab->counters = counters_new;
> -			slab_unlock(slab, &flags);
> +			slab_unlock(slab);
>  			return true;
>  		}
> -		slab_unlock(slab, &flags);
> +		slab_unlock(slab);
>  	}
>  
>  	cpu_relax();
> @@ -540,16 +525,16 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
>  		unsigned long flags;
>  
>  		local_irq_save(flags);
> -		__slab_lock(slab);
> +		slab_lock(slab);
>  		if (slab->freelist == freelist_old &&
>  					slab->counters == counters_old) {
>  			slab->freelist = freelist_new;
>  			slab->counters = counters_new;
> -			__slab_unlock(slab);
> +			slab_unlock(slab);
>  			local_irq_restore(flags);
>  			return true;
>  		}
> -		__slab_unlock(slab);
> +		slab_unlock(slab);
>  		local_irq_restore(flags);
>  	}
>  
> -- 
> 2.37.2

Otherwise looks good to me.

Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

-- 
Thanks,
Hyeonggon

Re: [PATCH v2 5/5] mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()

[Vlastimil Babka]

On 8/24/22 15:04, Hyeonggon Yoo wrote:
> On Tue, Aug 23, 2022 at 07:04:00PM +0200, Vlastimil Babka wrote:
>> The PREEMPT_RT specific disabling of irqs in __cmpxchg_double_slab()
>> (through slab_[un]lock()) is unnecessary as bit_spin_lock() disables
>> preemption and that's sufficient on RT where interrupts are threaded.
>> 
>> That means we no longer need the slab_[un]lock() wrappers, so delete
>> them and rename the current __slab_[un]lock() to slab_[un]lock().
>> 
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> Acked-by: David Rientjes <rientjes@google.com>
>> -
>>  /*
>>   * Interrupts must be disabled (for the fallback code to work right), typically
>> - * by an _irqsave() lock variant. Except on PREEMPT_RT where locks are different
>> - * so we disable interrupts as part of slab_[un]lock().
>> + * by an _irqsave() lock variant. Except on PREEMPT_RT where these variants do
>> + * not actually disable interrupts. On the other hand the migrate_disable()
> 
> You mean preempt_disable()?

I did, thanks for catching it.

> migrate_disable() will not be enough.
> 
>> -- 
>> 2.37.2
> 
> Otherwise looks good to me.
> 
> Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

Thanks!

Re: [PATCH v2 5/5] mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()

[Sebastian Andrzej Siewior]

On 2022-08-23 19:04:00 [+0200], Vlastimil Babka wrote:
> The PREEMPT_RT specific disabling of irqs in __cmpxchg_double_slab()
> (through slab_[un]lock()) is unnecessary as bit_spin_lock() disables
> preemption and that's sufficient on RT where interrupts are threaded.

maybe something like 
"… sufficient on PREEMPT_RT where no allocation/ free operation is
 performed in hardirq context and so interrupt the current operation."

> That means we no longer need the slab_[un]lock() wrappers, so delete
> them and rename the current __slab_[un]lock() to slab_[un]lock().
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: David Rientjes <rientjes@google.com>

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -454,7 +454,7 @@ static __always_inline void __slab_lock(struct slab *slab)
…
>  /*
>   * Interrupts must be disabled (for the fallback code to work right), typically
> - * by an _irqsave() lock variant. Except on PREEMPT_RT where locks are different
> - * so we disable interrupts as part of slab_[un]lock().
> + * by an _irqsave() lock variant. Except on PREEMPT_RT where these variants do
> + * not actually disable interrupts. On the other hand the migrate_disable()
> + * done by bit_spin_lock() is sufficient on PREEMPT_RT thanks to its threaded
> + * interrupts.

 "                                     On PREEMPT_RT the
 preempt_disable(), which is part of bit_spin_lock(), is sufficient
 because the policy is not to allow any allocation/ free operation in
 hardirq context. Therefore nothing can interrupt the operation."

This also includes SMP function calls (IPI).

>   */
>  static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
>  		void *freelist_old, unsigned long counters_old,

Sebastian

Re: [PATCH v2 5/5] mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()

[Vlastimil Babka]

On 8/24/22 18:25, Sebastian Andrzej Siewior wrote:
> On 2022-08-23 19:04:00 [+0200], Vlastimil Babka wrote:
>> The PREEMPT_RT specific disabling of irqs in __cmpxchg_double_slab()
>> (through slab_[un]lock()) is unnecessary as bit_spin_lock() disables
>> preemption and that's sufficient on RT where interrupts are threaded.
> 
> maybe something like 
> "… sufficient on PREEMPT_RT where no allocation/ free operation is
>  performed in hardirq context and so interrupt the current operation."
> 
>> That means we no longer need the slab_[un]lock() wrappers, so delete
>> them and rename the current __slab_[un]lock() to slab_[un]lock().
>> 
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> Acked-by: David Rientjes <rientjes@google.com>
> 
> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Thanks, incorporated your wording suggestions.

>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -454,7 +454,7 @@ static __always_inline void __slab_lock(struct slab *slab)
> …
>>  /*
>>   * Interrupts must be disabled (for the fallback code to work right), typically
>> - * by an _irqsave() lock variant. Except on PREEMPT_RT where locks are different
>> - * so we disable interrupts as part of slab_[un]lock().
>> + * by an _irqsave() lock variant. Except on PREEMPT_RT where these variants do
>> + * not actually disable interrupts. On the other hand the migrate_disable()
>> + * done by bit_spin_lock() is sufficient on PREEMPT_RT thanks to its threaded
>> + * interrupts.
> 
>  "                                     On PREEMPT_RT the
>  preempt_disable(), which is part of bit_spin_lock(), is sufficient
>  because the policy is not to allow any allocation/ free operation in
>  hardirq context. Therefore nothing can interrupt the operation."
> 
> This also includes SMP function calls (IPI).
> 
>>   */
>>  static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *slab,
>>  		void *freelist_old, unsigned long counters_old,
> 
> Sebastian

[PATCH 6/5] slub: Make PREEMPT_RT support less convoluted

[Sebastian Andrzej Siewior]

From: Thomas Gleixner <tglx@linutronix.de>

The slub code already has a few helpers depending on PREEMPT_RT. Add a few
more and get rid of the CONFIG_PREEMPT_RT conditionals all over the place.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: linux-mm@kvack.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---

Vlastimil, does it work for you to include this patch in your series? It
depends now on your series :) It has this USE_LOCKLESS_FAST_PATH() Linus
asked about so we should be good.

 mm/slub.c |   56 ++++++++++++++++++++++++--------------------------------
 1 file changed, 24 insertions(+), 32 deletions(-)

--- a/mm/slub.c
+++ b/mm/slub.c
@@ -104,9 +104,11 @@
  *   except the stat counters. This is a percpu structure manipulated only by
  *   the local cpu, so the lock protects against being preempted or interrupted
  *   by an irq. Fast path operations rely on lockless operations instead.
- *   On PREEMPT_RT, the local lock does not actually disable irqs (and thus
- *   prevent the lockless operations), so fastpath operations also need to take
- *   the lock and are no longer lockless.
+ *
+ *   On PREEMPT_RT, the local lock neither disables interrupts nor preemption
+ *   which means the lockless fastpath cannot be used as it might interfere with
+ *   an in-progress slow path operations. In this case the local lock is always
+ *   taken but it still utilizes the freelist for the common operations.
  *
  *   lockless fastpaths
  *
@@ -167,8 +169,9 @@
  * function call even on !PREEMPT_RT, use inline preempt_disable() there.
  */
 #ifndef CONFIG_PREEMPT_RT
-#define slub_get_cpu_ptr(var)	get_cpu_ptr(var)
-#define slub_put_cpu_ptr(var)	put_cpu_ptr(var)
+#define slub_get_cpu_ptr(var)		get_cpu_ptr(var)
+#define slub_put_cpu_ptr(var)		put_cpu_ptr(var)
+#define USE_LOCKLESS_FAST_PATH()	(true)
 #else
 #define slub_get_cpu_ptr(var)		\
 ({					\
@@ -180,6 +183,7 @@ do {					\
 	(void)(var);			\
 	migrate_enable();		\
 } while (0)
+#define USE_LOCKLESS_FAST_PATH()	(false)
 #endif
 
 #ifdef CONFIG_SLUB_DEBUG
@@ -474,7 +478,7 @@ static inline bool __cmpxchg_double_slab
 		void *freelist_new, unsigned long counters_new,
 		const char *n)
 {
-	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+	if (USE_LOCKLESS_FAST_PATH())
 		lockdep_assert_irqs_disabled();
 #if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
     defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
@@ -3287,14 +3291,8 @@ static __always_inline void *slab_alloc_
 
 	object = c->freelist;
 	slab = c->slab;
-	/*
-	 * We cannot use the lockless fastpath on PREEMPT_RT because if a
-	 * slowpath has taken the local_lock_irqsave(), it is not protected
-	 * against a fast path operation in an irq handler. So we need to take
-	 * the slow path which uses local_lock. It is still relatively fast if
-	 * there is a suitable cpu freelist.
-	 */
-	if (IS_ENABLED(CONFIG_PREEMPT_RT) ||
+
+	if (!USE_LOCKLESS_FAST_PATH() ||
 	    unlikely(!object || !slab || !node_match(slab, node))) {
 		object = __slab_alloc(s, gfpflags, node, addr, c);
 	} else {
@@ -3554,6 +3552,7 @@ static __always_inline void do_slab_free
 	void *tail_obj = tail ? : head;
 	struct kmem_cache_cpu *c;
 	unsigned long tid;
+	void **freelist;
 
 redo:
 	/*
@@ -3568,9 +3567,13 @@ static __always_inline void do_slab_free
 	/* Same with comment on barrier() in slab_alloc_node() */
 	barrier();
 
-	if (likely(slab == c->slab)) {
-#ifndef CONFIG_PREEMPT_RT
-		void **freelist = READ_ONCE(c->freelist);
+	if (unlikely(slab != c->slab)) {
+		__slab_free(s, slab, head, tail_obj, cnt, addr);
+		return;
+	}
+
+	if (USE_LOCKLESS_FAST_PATH()) {
+		freelist = READ_ONCE(c->freelist);
 
 		set_freepointer(s, tail_obj, freelist);
 
@@ -3582,16 +3585,8 @@ static __always_inline void do_slab_free
 			note_cmpxchg_failure("slab_free", s, tid);
 			goto redo;
 		}
-#else /* CONFIG_PREEMPT_RT */
-		/*
-		 * We cannot use the lockless fastpath on PREEMPT_RT because if
-		 * a slowpath has taken the local_lock_irqsave(), it is not
-		 * protected against a fast path operation in an irq handler. So
-		 * we need to take the local_lock. We shouldn't simply defer to
-		 * __slab_free() as that wouldn't use the cpu freelist at all.
-		 */
-		void **freelist;
-
+	} else {
+		/* Update the free list under the local lock */
 		local_lock(&s->cpu_slab->lock);
 		c = this_cpu_ptr(s->cpu_slab);
 		if (unlikely(slab != c->slab)) {
@@ -3606,11 +3601,8 @@ static __always_inline void do_slab_free
 		c->tid = next_tid(tid);
 
 		local_unlock(&s->cpu_slab->lock);
-#endif
-		stat(s, FREE_FASTPATH);
-	} else
-		__slab_free(s, slab, head, tail_obj, cnt, addr);
-
+	}
+	stat(s, FREE_FASTPATH);
 }
 
 static __always_inline void slab_free(struct kmem_cache *s, struct slab *slab,

Re: [PATCH 6/5] slub: Make PREEMPT_RT support less convoluted

[Vlastimil Babka]

On 8/25/22 09:51, Sebastian Andrzej Siewior wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> The slub code already has a few helpers depending on PREEMPT_RT. Add a few
> more and get rid of the CONFIG_PREEMPT_RT conditionals all over the place.
> 
> No functional change.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: linux-mm@kvack.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> 
> Vlastimil, does it work for you to include this patch in your series? It
> depends now on your series :) It has this USE_LOCKLESS_FAST_PATH() Linus
> asked about so we should be good.

Sure, I'll add it, thanks!

> 
>  mm/slub.c |   56 ++++++++++++++++++++++++--------------------------------
>  1 file changed, 24 insertions(+), 32 deletions(-)
> 
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -104,9 +104,11 @@
>   *   except the stat counters. This is a percpu structure manipulated only by
>   *   the local cpu, so the lock protects against being preempted or interrupted
>   *   by an irq. Fast path operations rely on lockless operations instead.
> - *   On PREEMPT_RT, the local lock does not actually disable irqs (and thus
> - *   prevent the lockless operations), so fastpath operations also need to take
> - *   the lock and are no longer lockless.
> + *
> + *   On PREEMPT_RT, the local lock neither disables interrupts nor preemption
> + *   which means the lockless fastpath cannot be used as it might interfere with
> + *   an in-progress slow path operations. In this case the local lock is always
> + *   taken but it still utilizes the freelist for the common operations.
>   *
>   *   lockless fastpaths
>   *
> @@ -167,8 +169,9 @@
>   * function call even on !PREEMPT_RT, use inline preempt_disable() there.
>   */
>  #ifndef CONFIG_PREEMPT_RT
> -#define slub_get_cpu_ptr(var)	get_cpu_ptr(var)
> -#define slub_put_cpu_ptr(var)	put_cpu_ptr(var)
> +#define slub_get_cpu_ptr(var)		get_cpu_ptr(var)
> +#define slub_put_cpu_ptr(var)		put_cpu_ptr(var)
> +#define USE_LOCKLESS_FAST_PATH()	(true)
>  #else
>  #define slub_get_cpu_ptr(var)		\
>  ({					\
> @@ -180,6 +183,7 @@ do {					\
>  	(void)(var);			\
>  	migrate_enable();		\
>  } while (0)
> +#define USE_LOCKLESS_FAST_PATH()	(false)
>  #endif
>  
>  #ifdef CONFIG_SLUB_DEBUG
> @@ -474,7 +478,7 @@ static inline bool __cmpxchg_double_slab
>  		void *freelist_new, unsigned long counters_new,
>  		const char *n)
>  {
> -	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +	if (USE_LOCKLESS_FAST_PATH())
>  		lockdep_assert_irqs_disabled();
>  #if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
>      defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
> @@ -3287,14 +3291,8 @@ static __always_inline void *slab_alloc_
>  
>  	object = c->freelist;
>  	slab = c->slab;
> -	/*
> -	 * We cannot use the lockless fastpath on PREEMPT_RT because if a
> -	 * slowpath has taken the local_lock_irqsave(), it is not protected
> -	 * against a fast path operation in an irq handler. So we need to take
> -	 * the slow path which uses local_lock. It is still relatively fast if
> -	 * there is a suitable cpu freelist.
> -	 */
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT) ||
> +
> +	if (!USE_LOCKLESS_FAST_PATH() ||
>  	    unlikely(!object || !slab || !node_match(slab, node))) {
>  		object = __slab_alloc(s, gfpflags, node, addr, c);
>  	} else {
> @@ -3554,6 +3552,7 @@ static __always_inline void do_slab_free
>  	void *tail_obj = tail ? : head;
>  	struct kmem_cache_cpu *c;
>  	unsigned long tid;
> +	void **freelist;
>  
>  redo:
>  	/*
> @@ -3568,9 +3567,13 @@ static __always_inline void do_slab_free
>  	/* Same with comment on barrier() in slab_alloc_node() */
>  	barrier();
>  
> -	if (likely(slab == c->slab)) {
> -#ifndef CONFIG_PREEMPT_RT
> -		void **freelist = READ_ONCE(c->freelist);
> +	if (unlikely(slab != c->slab)) {
> +		__slab_free(s, slab, head, tail_obj, cnt, addr);
> +		return;
> +	}
> +
> +	if (USE_LOCKLESS_FAST_PATH()) {
> +		freelist = READ_ONCE(c->freelist);
>  
>  		set_freepointer(s, tail_obj, freelist);
>  
> @@ -3582,16 +3585,8 @@ static __always_inline void do_slab_free
>  			note_cmpxchg_failure("slab_free", s, tid);
>  			goto redo;
>  		}
> -#else /* CONFIG_PREEMPT_RT */
> -		/*
> -		 * We cannot use the lockless fastpath on PREEMPT_RT because if
> -		 * a slowpath has taken the local_lock_irqsave(), it is not
> -		 * protected against a fast path operation in an irq handler. So
> -		 * we need to take the local_lock. We shouldn't simply defer to
> -		 * __slab_free() as that wouldn't use the cpu freelist at all.
> -		 */
> -		void **freelist;
> -
> +	} else {
> +		/* Update the free list under the local lock */
>  		local_lock(&s->cpu_slab->lock);
>  		c = this_cpu_ptr(s->cpu_slab);
>  		if (unlikely(slab != c->slab)) {
> @@ -3606,11 +3601,8 @@ static __always_inline void do_slab_free
>  		c->tid = next_tid(tid);
>  
>  		local_unlock(&s->cpu_slab->lock);
> -#endif
> -		stat(s, FREE_FASTPATH);
> -	} else
> -		__slab_free(s, slab, head, tail_obj, cnt, addr);
> -
> +	}
> +	stat(s, FREE_FASTPATH);
>  }
>  
>  static __always_inline void slab_free(struct kmem_cache *s, struct slab *slab,

Re: [PATCH 6/5] slub: Make PREEMPT_RT support less convoluted

[Hyeonggon Yoo]

On Thu, Aug 25, 2022 at 09:51:36AM +0200, Sebastian Andrzej Siewior wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> The slub code already has a few helpers depending on PREEMPT_RT. Add a few
> more and get rid of the CONFIG_PREEMPT_RT conditionals all over the place.
> 
> No functional change.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: linux-mm@kvack.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> 
> Vlastimil, does it work for you to include this patch in your series? It
> depends now on your series :) It has this USE_LOCKLESS_FAST_PATH() Linus
> asked about so we should be good.
> 
>  mm/slub.c |   56 ++++++++++++++++++++++++--------------------------------
>  1 file changed, 24 insertions(+), 32 deletions(-)
> 
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -104,9 +104,11 @@
>   *   except the stat counters. This is a percpu structure manipulated only by
>   *   the local cpu, so the lock protects against being preempted or interrupted
>   *   by an irq. Fast path operations rely on lockless operations instead.
> - *   On PREEMPT_RT, the local lock does not actually disable irqs (and thus
> - *   prevent the lockless operations), so fastpath operations also need to take
> - *   the lock and are no longer lockless.
> + *
> + *   On PREEMPT_RT, the local lock neither disables interrupts nor preemption
> + *   which means the lockless fastpath cannot be used as it might interfere with
> + *   an in-progress slow path operations. In this case the local lock is always
> + *   taken but it still utilizes the freelist for the common operations.

Thank you for correction!

>   *
>   *   lockless fastpaths
>   *
> @@ -167,8 +169,9 @@
>   * function call even on !PREEMPT_RT, use inline preempt_disable() there.
>   */
>  #ifndef CONFIG_PREEMPT_RT
> -#define slub_get_cpu_ptr(var)	get_cpu_ptr(var)
> -#define slub_put_cpu_ptr(var)	put_cpu_ptr(var)
> +#define slub_get_cpu_ptr(var)		get_cpu_ptr(var)
> +#define slub_put_cpu_ptr(var)		put_cpu_ptr(var)
> +#define USE_LOCKLESS_FAST_PATH()	(true)
>  #else
>  #define slub_get_cpu_ptr(var)		\
>  ({					\
> @@ -180,6 +183,7 @@ do {					\
>  	(void)(var);			\
>  	migrate_enable();		\
>  } while (0)
> +#define USE_LOCKLESS_FAST_PATH()	(false)
>  #endif
>  
>  #ifdef CONFIG_SLUB_DEBUG
> @@ -474,7 +478,7 @@ static inline bool __cmpxchg_double_slab
>  		void *freelist_new, unsigned long counters_new,
>  		const char *n)
>  {
> -	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> +	if (USE_LOCKLESS_FAST_PATH())
>  		lockdep_assert_irqs_disabled();
>  #if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
>      defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
> @@ -3287,14 +3291,8 @@ static __always_inline void *slab_alloc_
>  
>  	object = c->freelist;
>  	slab = c->slab;
> -	/*
> -	 * We cannot use the lockless fastpath on PREEMPT_RT because if a
> -	 * slowpath has taken the local_lock_irqsave(), it is not protected
> -	 * against a fast path operation in an irq handler. So we need to take
> -	 * the slow path which uses local_lock. It is still relatively fast if
> -	 * there is a suitable cpu freelist.
> -	 */
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT) ||
> +
> +	if (!USE_LOCKLESS_FAST_PATH() ||
>  	    unlikely(!object || !slab || !node_match(slab, node))) {
>  		object = __slab_alloc(s, gfpflags, node, addr, c);
>  	} else {
> @@ -3554,6 +3552,7 @@ static __always_inline void do_slab_free
>  	void *tail_obj = tail ? : head;
>  	struct kmem_cache_cpu *c;
>  	unsigned long tid;
> +	void **freelist;
>  
>  redo:
>  	/*
> @@ -3568,9 +3567,13 @@ static __always_inline void do_slab_free
>  	/* Same with comment on barrier() in slab_alloc_node() */
>  	barrier();
>  
> -	if (likely(slab == c->slab)) {
> -#ifndef CONFIG_PREEMPT_RT
> -		void **freelist = READ_ONCE(c->freelist);
> +	if (unlikely(slab != c->slab)) {
> +		__slab_free(s, slab, head, tail_obj, cnt, addr);
> +		return;
> +	}
> +
> +	if (USE_LOCKLESS_FAST_PATH()) {
> +		freelist = READ_ONCE(c->freelist);
>  
>  		set_freepointer(s, tail_obj, freelist);
>  
> @@ -3582,16 +3585,8 @@ static __always_inline void do_slab_free
>  			note_cmpxchg_failure("slab_free", s, tid);
>  			goto redo;
>  		}
> -#else /* CONFIG_PREEMPT_RT */
> -		/*
> -		 * We cannot use the lockless fastpath on PREEMPT_RT because if
> -		 * a slowpath has taken the local_lock_irqsave(), it is not
> -		 * protected against a fast path operation in an irq handler. So
> -		 * we need to take the local_lock. We shouldn't simply defer to
> -		 * __slab_free() as that wouldn't use the cpu freelist at all.
> -		 */
> -		void **freelist;
> -
> +	} else {
> +		/* Update the free list under the local lock */
>  		local_lock(&s->cpu_slab->lock);
>  		c = this_cpu_ptr(s->cpu_slab);
>  		if (unlikely(slab != c->slab)) {
> @@ -3606,11 +3601,8 @@ static __always_inline void do_slab_free
>  		c->tid = next_tid(tid);
>  
>  		local_unlock(&s->cpu_slab->lock);
> -#endif
> -		stat(s, FREE_FASTPATH);
> -	} else
> -		__slab_free(s, slab, head, tail_obj, cnt, addr);
> -
> +	}
> +	stat(s, FREE_FASTPATH);
>  }
>  
>  static __always_inline void slab_free(struct kmem_cache *s, struct slab *slab,

I have no strong opinion on its naming, but from view of correctness:

Looks good to me.
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>

-- 
Thanks,
Hyeonggon

Re: [PATCH v2 0/5] mm/slub: fix validation races and cleanup locking

[Vlastimil Babka]

On 8/23/22 19:03, Vlastimil Babka wrote:
> This series builds on the validation races fix posted previously [1]
> that became patch 2 here and contains all the details in its
> description.
> 
> Thanks to Hyeonggon Yoo's observation, patch 3 removes more slab_lock()
> usage that became unnecessary after patch 2.
> 
> This made it possible to further simplify locking code in patches 4 and
> 5. Since those are related to PREEMPT_RT, I'm CCing relevant people on
> this series.
> 
> Changes since v1 [2]:
> 
> - add acks/reviews from Hyeonggon and David
> - minor fixes to patch 2 as reported by Hyeonggon
> - patch 5 reworked to rely on disabled preemption by bit_spin_lock()
>   which should be sufficient without disabled interrupts on RT
> 
> git version:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=slub-validate-fix-v2r2
> 
> I plan to add this series to slab.git for-next in few days.

Thanks for the reviews, fixup suggestions and patch 6/5, now all pushed to
slab.git for-6.1/slub_validation_locking and merged to for-next.

> 
> [1] https://lore.kernel.org/all/20220809140043.9903-1-vbabka@suse.cz/
> [2] https://lore.kernel.org/all/20220812091426.18418-1-vbabka@suse.cz/
> 
> Vlastimil Babka (5):
>   mm/slub: move free_debug_processing() further
>   mm/slub: restrict sysfs validation to debug caches and make it safe
>   mm/slub: remove slab_lock() usage for debug operations
>   mm/slub: convert object_map_lock to non-raw spinlock
>   mm/slub: simplify __cmpxchg_double_slab() and slab_[un]lock()
> 
>  mm/slub.c | 417 ++++++++++++++++++++++++++++++++----------------------
>  1 file changed, 251 insertions(+), 166 deletions(-)
>