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Date: Thu, 18 Nov 2010 13:36:08 -0800 (PST)
From: David Rientjes <rientjes@google.com>
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To: Matt Mackall <mpm@selenic.com>
cc: b32542@freescale.com, linux-mm@kvack.org, cl@linux-foundation.org,
        penberg@cs.helsinki.fi, tytso@mit.edu, linux-kernel@vger.kernel.org,
        Zeng Zhaoming <zengzm.kernel@gmail.com>
Subject: Re: [PATCH] slub: operate cache name memory same to slab and slob
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On Thu, 18 Nov 2010, Matt Mackall wrote:

> > Get a memory leak complaint about ext4:
> >   comm "mount", pid 1159, jiffies 4294904647 (age 6077.804s)
> >   hex dump (first 32 bytes):
> >     65 78 74 34 5f 67 72 6f 75 70 69 6e 66 6f 5f 31  ext4_groupinfo_1
> >     30 00 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5  0.kkkkkkkkkkkkk.
> >   backtrace:
> >     [<c068ade3>] kmemleak_alloc+0x93/0xd0
> >     [<c024e54c>] __kmalloc_track_caller+0x30c/0x380
> >     [<c02269d3>] kstrdup+0x33/0x60
> >     [<c0318a70>] ext4_mb_init+0x4e0/0x550
> >     [<c0304e0e>] ext4_fill_super+0x1e6e/0x2f60
> >     [<c0261140>] mount_bdev+0x1c0/0x1f0
> >     [<c02fc00f>] ext4_mount+0x1f/0x30
> >     [<c02603d8>] vfs_kern_mount+0x78/0x250
> >     [<c026060e>] do_kern_mount+0x3e/0x100
> >     [<c027b4c2>] do_mount+0x2e2/0x780
> >     [<c027ba04>] sys_mount+0xa4/0xd0
> >     [<c010429f>] sysenter_do_call+0x12/0x38
> >     [<ffffffff>] 0xffffffff
> > 
> > It is cause by slub manage the cache name different from slab and slob.
> > In slab and slob, only reference to name, alloc and reclaim the memory
> > is the duty of the code that invoked kmem_cache_create().
> > 
> > In slub, cache name duplicated when create. This ambiguity will cause
> > some memory leaks and double free if kmem_cache_create() pass a
> > dynamic malloc cache name.
> 
> I don't get it.
> 
> Caller allocates X, passes X to slub, slub duplicates X as X', and
> properly frees X', then caller frees X. Yes, that's silly, but where's
> the leak?
> 

The leak in ext4_mb_init() above is because it is using kstrdup() to 
allocate the string itself and then on destroy uses kmem_cache_name() to 
attain the slub allocator's pointer to the name, not the memory the ext4 
layer allocated itself.
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