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

[Shakeel Butt]

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@xxxxxxxxxx
> 
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/harry/linux.git/log/?h=kmalloc-no-objext-v3r1
> 
> To: Vlastimil Babka <vbabka@xxxxxxxxxx>
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> To: Hao Li <hao.li@xxxxxxxxx>
> To: Christoph Lameter <cl@xxxxxxxxxx>
> To: David Rientjes <rientjes@xxxxxxxxxx>
> To: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> To: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> To: Hao Ge <hao.ge@xxxxxxxxx>
> To: Kees Cook <kees@xxxxxxxxxx>
> To: Pedro Falcato <pfalcato@xxxxxxx>
> To: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> To: Danielle Constantino <dcostantino@xxxxxxxx>
> To: Liam R. Howlett <liam@xxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>

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@xxxxxxxxx>