Re: [PATCH] mm: memcg: remove direct use of __memcg_kmem_uncharge_page

Matthew Wilcox
2023-12-13T15:01:49Z

On Wed, Dec 13, 2023 at 01:04:14PM +0000, Yosry Ahmed wrote:
> memcg_kmem_uncharge_page() is an inline wrapper around
> __memcg_kmem_uncharge_page() that checks memcg_kmem_online() before
> making the function call. Internally, __memcg_kmem_uncharge_page() has a
> folio_memcg_kmem() check.
>
> The only direct user of __memcg_kmem_uncharge_page(),
> free_pages_prepare(), checks PageMemcgKmem() before calling it to avoid
> the function call if possible. Move the folio_memcg_kmem() check from
> __memcg_kmem_uncharge_page() to memcg_kmem_uncharge_page() as
> PageMemcgKmem() -- which does the same thing under the hood. Now
> free_pages_prepare() can also use memcg_kmem_uncharge_page().

I think you've just pessimised all the other places which call
memcg_kmem_uncharge_page(). It's a matter of probabilities. In
free_pages_prepare(), most of the pages being freed are not accounted
to memcg. Whereas in fork() we are absolutely certain that the pages
were accounted because we accounted them.

I think this is a bad change.