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

Paul E. McKenney
2020-12-09T19:42:44Z

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