linux-kernel - Re: [PATCH 04/24] mm/swap: avoid setting page lock bit and doing extra unlock check

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Date:   Sun, 19 Nov 2023 20:17:59 -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 04/24] mm/swap: avoid setting page lock bit and doing
 extra unlock check

On Sun, Nov 19, 2023 at 11:48 AM Kairui Song <ryncsn@...il.com> wrote:
>
> From: Kairui Song <kasong@...cent.com>
>
> When swapping in a page, mem_cgroup_swapin_charge_folio is called for new
> allocated folio, nothing else is referencing the folio so no need to set
> the lock bit. This avoided doing unlock check on error path.
>
> Signed-off-by: Kairui Song <kasong@...cent.com>
> ---
>  mm/swap_state.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index ac4fa404eaa7..45dd8b7c195d 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -458,6 +458,8 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,

You move the mem_cgroup_swapin_charge_folio() inside the for loop:


        for (;;) {
                int err;
                /*
                 * First check the swap cache.  Since this is normally
                 * called after swap_cache_get_folio() failed, re-calling
                 * that would confuse statistics.
                 */
                folio = filemap_get_folio(swap_address_space(entry),
                                                swp_offset(entry));


>                                                 mpol, ilx, numa_node_id());
>                 if (!folio)
>                          goto fail_put_swap;
> +               if (mem_cgroup_swapin_charge_folio(folio, NULL, gfp_mask, entry))
> +                       goto fail_put_folio;

Wouldn't it cause repeat charging of the folio when it is racing
against others in the for loop?

>
>                 /*
>                  * Swap entry may have been freed since our caller observed it.
> @@ -483,13 +485,9 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>         /*
>          * The swap entry is ours to swap in. Prepare the new page.
>          */
> -
>         __folio_set_locked(folio);
>         __folio_set_swapbacked(folio);
>
> -       if (mem_cgroup_swapin_charge_folio(folio, NULL, gfp_mask, entry))
> -               goto fail_unlock;
> -

The original code makes the charge outside of the for loop. Only the
winner can charge once.

Chris