Re: [PATCH] mm/slab: don't set SLAB_MAY_ACCOUNT for KMALLOC_PARTITION_CACHES

[Hao Li <hao.li@linux.dev>]

On Wed, Nov 11, 2026 at 02:08:40PM +0100, Vlastimil Babka (SUSE) wrote:
> On 11/11/26 13:49, Hao Li wrote:
> > An allocation with __GFP_ACCOUNT set (without __GFP_DMA or
> > __GFP_RECLAIMABLE) is redirected to KMALLOC_CGROUP and will never hit
> > KMALLOC_NORMAL or KMALLOC_PARTITION_CACHES. Consequently, neither of
> > these caches requires objcg slots in obj_exts.
> > 
> > While KMALLOC_NORMAL already skips setting SLAB_MAY_ACCOUNT,
> > KMALLOC_PARTITION_CACHES still carries it unnecessarily.
> > 
> > Extend the check to skip KMALLOC_PARTITION_CACHES as well, saving
> > unnecessary objcg slots in obj_exts.
> > 
> > Signed-off-by: Hao Li <hao.li@linux.dev>
> 
> Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

Thanks!

> 
> > ---
> > This doesn't seem like a real bug to me, just a small waste of memory, so I
> > didn't add a Fixes tag.
> 
> Hmm I'd add
> Fixes: 7def2e8549e5 ("mm/slab: add cache_ and slab_needs_objcg() helpers")

Ah right, I mixed up Fixes with cc stable. Thanks for pointing it out!

> 
> The question is whether cc stable as well. 7.3 LTS will be used for Android
> and IIRC they have memory allocation profiling enabled. Not sure about
> kmalloc partitions. Cc'ing Suren.

Makes sense. If Android uses partitioned caches, then it looks like we should
cc stable.

> 
> > ---
> >  mm/slab_common.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 2befd24b4d09..f0ea1fa178e7 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -1000,16 +1000,18 @@ new_kmalloc_cache(int idx, enum kmalloc_cache_type type)
> >  #endif
> >  
> >  	/*
> > -	 * If memcg_kmem is enabled and this is a KMALLOC_NORMAL cache and not
> > -	 * aliased with any other type, make sure it's never merged with any other
> > -	 * cache.
> > +	 * If memcg_kmem is enabled and this is a KMALLOC_NORMAL or
> > +	 * KMALLOC_PARTITION_CACHES cache and not aliased with any other type,
> > +	 * make sure it's never merged with any other cache.
> >  	 *
> >  	 * In other cases the kmalloc cache may end up being used for a
> >  	 * __GFP_ACCOUNT allocation so mark it as such. The exception is a
> >  	 * KMALLOC_NO_OBJ_EXT cache.
> >  	 */
> >  	if (!mem_cgroup_kmem_disabled()) {
> > -		if (type == KMALLOC_NORMAL && KMALLOC_RECLAIM != KMALLOC_NORMAL)
> > +		if (type >= KMALLOC_PARTITION_START &&
> > +		    type <= KMALLOC_PARTITION_END &&
> > +		    KMALLOC_RECLAIM != KMALLOC_NORMAL)
> >  			flags |= SLAB_NO_MERGE;
> >  		else if (!(flags & SLAB_NO_OBJ_EXT))
> >  			flags |= SLAB_MAY_ACCOUNT;
>