linux-kernel - Re: CONFIG_DEBUG_SLAB_LEAK omits size-4096 and larger?

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Date:	Wed, 11 Jun 2008 16:57:50 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Jeff Layton <jlayton@...chiereds.net>
Cc:	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org,
	"Weathers, Norman R." <Norman.R.Weathers@...ocophillips.com>
Subject: Re: CONFIG_DEBUG_SLAB_LEAK omits size-4096 and larger?

On Wed, Jun 11, 2008 at 04:09:47PM -0400, Jeff Layton wrote:
> On Wed, 11 Jun 2008 15:52:22 -0400
> "J. Bruce Fields" <bfields@...ldses.org> wrote:
> 
> > I'm probably missing something fundamental--why doesn't
> > /proc/slab_allocators show any results for size-x where x >= 4096?
> > 
> > Someone's seeing a performance problem with the linux nfs server.  One
> > of the symptoms is the "size-4096" slab cache seems to be out of
> > control.  I assumed that meant that memory allocated by kmalloc() might
> > be leaking, so figured it might be interesting to turn on
> > CONFIG_DEBUG_SLAB_LEAK.  As far as I can tell what that does is list
> > kmalloc() callers in /proc/slab_allocators.  But that doesn't seem to be
> > showing any results for size-4096.  Can anyone provide a clue?
> > Thanks!
> > 
> > --b.
> > 
> 
> 
> Hmm...I've never used this, but in kmem_cache_alloc():
> 
>         /*
>          * Enable redzoning and last user accounting, except for caches with
>          * large objects, if the increased size would increase the object size
>          * above the next power of two: caches with object sizes just above a
>          * power of two have a significant amount of internal fragmentation.
>          */
>         if (size < 4096 || fls(size - 1) == fls(size-1 + REDZONE_ALIGN +
>                                                 2 * sizeof(unsigned long long)))
>                 flags |= SLAB_RED_ZONE | SLAB_STORE_USER;
> 
> 
> ...looks like it specifically excludes some caches.

Ah, I missed that!  I'm a little confused as to how those flags behavior
affect the collection of the leak debugging data, but I can verify that
the below does result in size-4096 showing up in /proc/slab_allocators;
hopefully there's no more negative result than the performance penalty.

Norman, do you think you could try applying this and then trying again?

--b.


diff --git a/mm/slab.c b/mm/slab.c
index 06236e4..b379e31 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2202,7 +2202,7 @@ kmem_cache_create (const char *name, size_t size, size_t align,
 	 * above the next power of two: caches with object sizes just above a
 	 * power of two have a significant amount of internal fragmentation.
 	 */
-	if (size < 4096 || fls(size - 1) == fls(size-1 + REDZONE_ALIGN +
+	if (size < 8192 || fls(size - 1) == fls(size-1 + REDZONE_ALIGN +
 						2 * sizeof(unsigned long long)))
 		flags |= SLAB_RED_ZONE | SLAB_STORE_USER;
 	if (!(flags & SLAB_DESTROY_BY_RCU))
--
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