Re: [PATCH slab/for-next-fixes v3 0/4] mm/slab: fix unbounded recursion in free path with memalloc profiling

[Harry Yoo <harry@kernel.org>]

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On 7/17/26 4:37 AM, Shakeel Butt wrote:
> On Mon, Jul 13, 2026 at 11:28:48PM +0900, Harry Yoo (Oracle) wrote:
>> This is a follow-up fix after the recent discussion [1].
>> See patch 4 for the detailed description on the bug.
>>
>> Based on slab/for-next-fixes (af9ea231c0b45) and is available at
>> git.kernel.org [2].
>>
>> Instead preventing cycles by bumping up the allocation size of obj_exts
>> arrays, it introduces a new kmalloc type called KMALLOC_NO_OBJ_EXT and
>> disallow formation of cycles between kmalloc types when allocating
>> obj_exts arrays. obj_exts arrays of normal kmalloc caches are served
>> from KMALLOC_NO_OBJ_EXT caches (that don't have obj_exts), and all other
>> obj_exts arrays are served from normal kmalloc caches.
>>
>> I tried to reuse SLAB_ALLOC_NO_RECURSE to make kmalloc_slab() select
>> KMALLOC_NO_OBJ_EXT, but it was not great because it does not allow
>> sheaves for those caches. So I introduced a new slab alloc flag
>> SLAB_ALLOC_NO_OBJ_EXT.
>>
>> To avoid huge confusion, I had to decouple "disallowing sheaves"
>> semantics from SLAB_NO_OBJ_EXT and introduced SLAB_NO_SHEAVES.
>>
>> While this cannot be directly backported to v6.18 and v6.12 due to lack
>> of SLAB_ALLOC_* flags and kmalloc_flags(), I don't this will be
>> particularily challenging to backport it. Instead of a new slab alloc
>> flag, we can use __GFP_NO_OBJ_EXT to select KMALLOC_NO_OBJ_EXT as
>> kmalloc caches don't have sheaves in v6.18 anyway.
>>
>> [1] https://lore.kernel.org/linux-mm/9a139365-28e6-4f1e-b35b-7f6091e9aa14@kernel.org
>>
>> [2] https://git.kernel.org/pub/scm/linux/kernel/git/harry/linux.git/log/?h=kmalloc-no-objext-v3r1
>>
>> To: Vlastimil Babka <vbabka@kernel.org>
>> To: Andrew Morton <akpm@linux-foundation.org>
>> To: Hao Li <hao.li@linux.dev>
>> To: Christoph Lameter <cl@gentwo.org>
>> To: David Rientjes <rientjes@google.com>
>> To: Roman Gushchin <roman.gushchin@linux.dev>
>> To: Suren Baghdasaryan <surenb@google.com>
>> To: Hao Ge <hao.ge@linux.dev>
>> To: Kees Cook <kees@kernel.org>
>> To: Pedro Falcato <pfalcato@suse.de>
>> To: Shakeel Butt <shakeel.butt@linux.dev>
>> To: Danielle Constantino <dcostantino@meta.com>
>> To: Liam R. Howlett <liam@infradead.org>
>> Cc: linux-mm@kvack.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
> 
> I tested next-20260716 which has this series against next-20260707 which does
> not have the fix. The reproducer were able to trigger the leak/recursion on
> next-20260707 but not on next-20260716. So, you can add:
> 
> Tested-by: Shakeel Butt <shakeel.butt@linux.dev>

Thanks a lot for testing, Shakeel!

-- 
Cheers,
Harry / Hyeonggon


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