Re: [PATCH] mm/slab: fix an incorrect check in obj_exts_alloc_size()

[Harry Yoo]

On Mon, Mar 09, 2026 at 03:00:17PM +0100, vbabka@xxxxxxxxxx wrote:
> On 3/9/26 08:22, Harry Yoo wrote:
> > obj_exts_alloc_size() prevents recursive allocation of slabobj_ext
> > array from the same cache, to avoid creating slabs that are never freed.
> > 
> > There is one mistake that returns the original size when memory
> > allocation profiling is disabled. The assumption was that
> > memcg-triggered slabobj_ext allocation is always served from
> > KMALLOC_CGROUP type. But this is wrong [1]: when the caller specifies
> > both __GFP_RECLAIMABLE and __GFP_ACCOUNT with SLUB_TINY enabled, the
> > allocation is served from normal kmalloc. This is because kmalloc_type()
> > prioritizes __GFP_RECLAIMABLE over __GFP_ACCOUNT, and SLUB_TINY aliases
> > KMALLOC_RECLAIM with KMALLOC_NORMAL.
> 
> Hm that's suboptimal (leads to sparsely used obj_exts in normal kmalloc
> slabs) and maybe separately from this hotfix we could make sure that with
> SLUB_TINY, __GFP_ACCOUNT is preferred going forward?

To be honest, I don't a have strong opinion on that.

Is grouping by mobility (for anti-fragmentation less) important on
SLUB_TINY systems?

> > As a result, the recursion guard is bypassed and the problematic slabs
> > can be created. Fix this by removing the mem_alloc_profiling_enabled()
> > check entirely. The remaining is_kmalloc_normal() check is still
> > sufficient to detect whether the cache is of KMALLOC_NORMAL type and
> > avoid bumping the size if it's not.
> > 
> > Without SLUB_TINY, no functional change intended.
> > With SLUB_TINY, allocations with __GFP_ACCOUNT|__GFP_RECLAIMABLE
> > now allocate a larger array if the sizes equal.
> > 
> > Reported-by: Zw Tang <shicenci@xxxxxxxxx>
> > Fixes: 280ea9c3154b ("mm/slab: avoid allocating slabobj_ext array from its own slab")
> > Closes: https://lore.kernel.org/linux-mm/CAPHJ_VKuMKSke8b11AZQw1PTSFN4n2C0gFxC6xGOG0ZLHgPmnA@xxxxxxxxxxxxxx
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> 
> Added to slab/for-next-fixes, thanks!

Thanks!

-- 
Cheers,
Harry / Hyeonggon