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Date: Wed, 20 Nov 2013 17:47:12 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [git pull] vfs.git bits and pieces
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On Wed, Nov 20, 2013 at 05:42:11PM +0000, Al Viro wrote:
> [Apologies for delay; I'd spent the last day hunting down something that
> turned out to be a VM leak completely unrelated to this stuff - it's
> present in mainline, for starters.  Unreliable reproducers make for fun
> bisects ;-/  Anyway, by now I'm absolutely sure that this is a VM bug and
> not something I had somehow managed to break, so...]

BTW, something odd happened to mm/memory.c - either a mangled patch
or a lost followup.  Take a look at the last commit in there:
commit ea1e7ed33708c7a760419ff9ded0a6cb90586a50
Author: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Date:   Thu Nov 14 14:31:53 2013 -0800

    mm: create a separate slab for page->ptl allocation
    
    If DEBUG_SPINLOCK and DEBUG_LOCK_ALLOC are enabled spinlock_t on x86_64
    is 72 bytes.  For page->ptl they will be allocated from kmalloc-96 slab,
    so we loose 24 on each.  An average system can easily allocate few tens
    thousands of page->ptl and overhead is significant.
    
    Let's create a separate slab for page->ptl allocation to solve this.
    
    Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Ingo Molnar <mingo@elte.hu>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Fair enough, and yes, it does create that separate slab.  The problem is,
it's still using kmalloc/kfree for those beasts - page_ptl_cachep isn't
used at all...