Re: [PATCH] memcg: slub: fix SUnreclaim for post charged objects

[Vlastimil Babka <vbabka@suse.cz>]

On 12/10/24 05:06, Shakeel Butt wrote:
> Large kmalloc directly allocates from the page allocator and then use
> lruvec_stat_mod_folio() to increment the unreclaimable slab stats for
> global and memcg. However when post memcg charging of slab objects was
> added in commit 9028cdeb38e1 ("memcg: add charging of already allocated
> slab objects"), it missed to correctly handle the unreclaimable slab
> stats for memcg.
> 
> One user visisble effect of that bug is that the node level
> unreclaimable slab stat will work correctly but the memcg level stat can
> underflow as kernel correctly handles the free path but the charge path
> missed to increment the memcg level unreclaimable slab stat. Let's fix
> by correctly handle in the post charge code path.
> 
> Fixes: 9028cdeb38e1 ("memcg: add charging of already allocated slab objects")

That's a 6.12-rc1 commit so I'm adding cc stable.

> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>

Queued in slab/for-next-fixes, thanks!

Vlastimil

> ---
>  mm/slub.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index f62c829b7b6b..88bf2bf51bd6 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2189,9 +2189,24 @@ bool memcg_slab_post_charge(void *p, gfp_t flags)
>  
>  	folio = virt_to_folio(p);
>  	if (!folio_test_slab(folio)) {
> -		return folio_memcg_kmem(folio) ||
> -			(__memcg_kmem_charge_page(folio_page(folio, 0), flags,
> -						  folio_order(folio)) == 0);
> +		int size;
> +
> +		if (folio_memcg_kmem(folio))
> +			return true;
> +
> +		if (__memcg_kmem_charge_page(folio_page(folio, 0), flags,
> +					     folio_order(folio)))
> +			return false;
> +
> +		/*
> +		 * This folio has already been accounted in the global stats but
> +		 * not in the memcg stats. So, subtract from the global and use
> +		 * the interface which adds to both global and memcg stats.
> +		 */
> +		size = folio_size(folio);
> +		node_stat_mod_folio(folio, NR_SLAB_UNRECLAIMABLE_B, -size);
> +		lruvec_stat_mod_folio(folio, NR_SLAB_UNRECLAIMABLE_B, size);
> +		return true;
>  	}
>  
>  	slab = folio_slab(folio);