Re: [PATCH v2 1/2] mm, slab: move memcg charging to post-alloc hook

[Shakeel Butt]

On Mon, Mar 25, 2024 at 09:20:32AM +0100, Vlastimil Babka wrote:
> The MEMCG_KMEM integration with slab currently relies on two hooks
> during allocation. memcg_slab_pre_alloc_hook() determines the objcg and
> charges it, and memcg_slab_post_alloc_hook() assigns the objcg pointer
> to the allocated object(s).
> 
> As Linus pointed out, this is unnecessarily complex. Failing to charge
> due to memcg limits should be rare, so we can optimistically allocate
> the object(s) and do the charging together with assigning the objcg
> pointer in a single post_alloc hook. In the rare case the charging
> fails, we can free the object(s) back.
> 
> This simplifies the code (no need to pass around the objcg pointer) and
> potentially allows to separate charging from allocation in cases where
> it's common that the allocation would be immediately freed, and the
> memcg handling overhead could be saved.
> 
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Link: https://lore.kernel.org/all/CAHk-=whYOOdM7jWy5jdrAm8LxcgCMFyk2bt8fYYvZzM4U-zAQA@xxxxxxxxxxxxxx/
> Reviewed-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> Reviewed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>