Re: [RFC PATCH 3/3] mm: entirely remove lru_add_drain in do_swap_page

[Kairui Song]

On Thu, Jun 11, 2026 at 6:52 PM Barry Song (Xiaomi) <baohua@xxxxxxxxxx> wrote:
>
> We are doing a lot of redundant lru_add_drain() calls in
> do_swap_page(), especially for synchronous I/O devices. For
> example, the test program below currently ends up draining
> lru_cache 100% of the time:
>
> int main(int argc, char *argv[])
> {
>         int i;
>  #define SIZE 100*1024*1024
>         while(1) {
>                 volatile int *p = mmap(0, SIZE, PROT_READ | PROT_WRITE,
>                         MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>
>                 for (int i = 0; i < SIZE/sizeof(int); i++)
>                         p[i] =  i%64;
>                 madvise((void *)p, SIZE, MADV_PAGEOUT);
>                 for (int i = 0; i < SIZE/sizeof(int); i++)
>                         p[i] =  i%64;
>                 munmap(p, SIZE);
>         }
>         return 0;
> }
>
> Folio reuse now relies primarily on the exclusive hint, making
> lru_cache draining to drop the refcount in lru_cache largely
> irrelevant.
>
> Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> ---
>  mm/memory.c | 10 ----------
>  1 file changed, 10 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index ce8ef27e7a54..b5a78670bcc8 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4903,16 +4903,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>         } else if (folio != swapcache)
>                 page = folio_page(folio, 0);
>
> -       /*
> -        * If we want to map a page that's in the swapcache writable, we
> -        * have to detect via the refcount if we're really the exclusive
> -        * owner. Try removing the extra reference from the local LRU
> -        * caches if required.
> -        */
> -       if ((vmf->flags & FAULT_FLAG_WRITE) &&
> -           !folio_test_ksm(folio) && !folio_test_lru(folio))
> -               lru_add_drain();
> -
>         folio_throttle_swaprate(folio, GFP_KERNEL);
>
>         /*
> --
> 2.39.3 (Apple Git-146)
>
>

There is a ref check in should_try_to_free_swap though, perhaps we
should also improve that part too or we may have more more folios
stuck in swap cache after this change?