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Date: Thu, 18 Nov 2010 23:41:24 +0200
From: Pekka Enberg <penberg@cs.helsinki.fi>
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To: Matt Mackall <mpm@selenic.com>
CC: b32542@freescale.com, linux-mm@kvack.org, cl@linux-foundation.org,
        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
References: <1290049259-20108-1-git-send-email-b32542@freescale.com> <1290114908.26343.721.camel@calx>
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On 18.11.2010 23.15, Matt Mackall wrote:
> On Thu, 2010-11-18 at 11:00 +0800, b32542@freescale.com wrote:
>> From: Zeng Zhaoming<zengzm.kernel@gmail.com>
>>
>> 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?
>
> But slub and slab should obviously both manage names in the same way,
> namely the historical "caller allocates" way. So:
>
> Acked-by: Matt Mackall<mpm@selenic.com>

The kstrdup() is there because of SLUB cache merging. See commit 
84c1cf62465e2fb0a692620dcfeb52323ab03d48 ("SLUB: Fix merged slab cache 
names") for details.
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