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.
