linux-kernel - Re: [PATCH 15/24] mm/swap: avoid an duplicated swap cache lookup for SYNCHRONOUS

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Date:   Tue, 21 Nov 2023 09:15:50 -0800
From:   Chris Li <chrisl@...nel.org>
To:     Kairui Song <kasong@...cent.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        "Huang, Ying" <ying.huang@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 15/24] mm/swap: avoid an duplicated swap cache lookup for
 SYNCHRONOUS_IO device

On Sun, Nov 19, 2023 at 11:48 AM Kairui Song <ryncsn@...il.com> wrote:
>
> From: Kairui Song <kasong@...cent.com>
>
> When a xa_value is returned by the cache lookup, keep it to be used
> later for workingset refault check instead of doing the looking up again
> in swapin_no_readahead.
>
> This does have a side effect of making swapoff also triggers workingset
> check, but should be fine since swapoff does effect the workload in many
> ways already.

I need to sleep on it a bit to see if this will create another problem or not.

>
> Signed-off-by: Kairui Song <kasong@...cent.com>
> ---
>  mm/swap_state.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index e057c79fb06f..51de2a0412df 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -872,7 +872,6 @@ static struct page *swapin_no_readahead(swp_entry_t entry, gfp_t gfp_mask,
>  {
>         struct folio *folio;
>         struct page *page;
> -       void *shadow = NULL;
>
>         page = alloc_pages_mpol(gfp_mask, 0, mpol, ilx, numa_node_id());
>         folio = (struct folio *)page;
> @@ -888,10 +887,6 @@ static struct page *swapin_no_readahead(swp_entry_t entry, gfp_t gfp_mask,
>
>                 mem_cgroup_swapin_uncharge_swap(entry);
>
> -               shadow = get_shadow_from_swap_cache(entry);
> -               if (shadow)
> -                       workingset_refault(folio, shadow);
> -
>                 folio_add_lru(folio);
>
>                 /* To provide entry to swap_readpage() */
> @@ -922,11 +917,12 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
>         enum swap_cache_result cache_result;
>         struct swap_info_struct *si;
>         struct mempolicy *mpol;
> +       void *shadow = NULL;
>         struct folio *folio;
>         struct page *page;
>         pgoff_t ilx;
>
> -       folio = swap_cache_get_folio(entry, vmf, NULL);
> +       folio = swap_cache_get_folio(entry, vmf, &shadow);
>         if (folio) {
>                 page = folio_file_page(folio, swp_offset(entry));
>                 cache_result = SWAP_CACHE_HIT;
> @@ -938,6 +934,8 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
>         if (swap_use_no_readahead(si, swp_offset(entry))) {
>                 page = swapin_no_readahead(entry, gfp_mask, mpol, ilx, vmf->vma->vm_mm);
>                 cache_result = SWAP_CACHE_BYPASS;
> +               if (shadow)
> +                       workingset_refault(page_folio(page), shadow);

It is inconsistent why other flavors of readahead do not do the
workingset_refault here.
I suggest keeping the workingset_refault in swapin_no_readahead() and
pass the shadow argument in.

Chris

>         } else if (swap_use_vma_readahead(si)) {
>                 page = swap_vma_readahead(entry, gfp_mask, mpol, ilx, vmf);
>                 cache_result = SWAP_CACHE_MISS;
> --
> 2.42.0
>
>