Re: [RFC PATCH 1/3] mm/mincore, swap: consolidate swap cache checking for mincore

[Nhat Pham <nphamcs@gmail.com>]

On Thu, Aug 7, 2025 at 8:27 AM Kairui Song <ryncsn@gmail.com> wrote:
>
> From: Kairui Song <kasong@tencent.com>
>
> The filemap_get_incore_folio (previously find_get_incore_page) helper
> was introduced by commit 61ef18655704 ("mm: factor find_get_incore_page
> out of mincore_page") to be used by later commit f5df8635c5a3 ("mm: use
> find_get_incore_page in memcontrol"), so memory cgroup charge move code
> can be simplified.
>
> But commit 6b611388b626 ("memcg-v1: remove charge move code") removed
> that user completely, it's only used by mincore now.
>
> So this commit basically reverts commit 61ef18655704 ("mm: factor
> find_get_incore_page out of mincore_page"). Move it back to mincore side
> to simplify the code.
>
> Signed-off-by: Kairui Song <kasong@tencent.com>

Seems reasonable to me for the most part - just a couple of questions below.

> ---
>  mm/mincore.c    | 29 +++++++++++++++++++++++++++--
>  mm/swap.h       | 10 ----------
>  mm/swap_state.c | 38 --------------------------------------
>  3 files changed, 27 insertions(+), 50 deletions(-)
>
> diff --git a/mm/mincore.c b/mm/mincore.c
> index 10dabefc3acc..f0d3c9419e58 100644
> --- a/mm/mincore.c
> +++ b/mm/mincore.c
> @@ -64,8 +64,33 @@ static unsigned char mincore_page(struct address_space *mapping, pgoff_t index)
>          * any other file mapping (ie. marked !present and faulted in with
>          * tmpfs's .fault). So swapped out tmpfs mappings are tested here.
>          */
> -       folio = filemap_get_incore_folio(mapping, index);
> -       if (!IS_ERR(folio)) {
> +       if (IS_ENABLED(CONFIG_SWAP) && shmem_mapping(mapping)) {

Do we need CONFIG_SWAP check here? I suppose if !CONFIG_SWAP we'll
never end up with an ordinary swap entry stored here right?

Saves a couple of cycles, I suppose. No strong opinions.

> +               folio = filemap_get_entry(mapping, index);
> +               /*
> +                * shmem/tmpfs may return swap: account for swapcache
> +                * page too.
> +                */
> +               if (xa_is_value(folio)) {
> +                       struct swap_info_struct *si;
> +                       swp_entry_t swp = radix_to_swp_entry(folio);
> +                       /* There might be swapin error entries in shmem mapping. */
> +                       if (non_swap_entry(swp))
> +                               return 0;
> +                       /* Prevent swap device to being swapoff under us */
> +                       si = get_swap_device(swp);
> +                       if (si) {
> +                               folio = filemap_get_folio(swap_address_space(swp),
> +                                                         swap_cache_index(swp));
> +                               put_swap_device(si);
> +                       } else {
> +                               return 0;
> +                       }
> +               }
> +       } else {
> +               folio = filemap_get_folio(mapping, index);
> +       }
> +
> +       if (folio) {

Should this check be "if (!IS_ERR(folio))"? Seems like that's how we
inspect the output of filemap_get_folio() in other locations (for e.g,
in filemap_fault()).

>                 present = folio_test_uptodate(folio);
>                 folio_put(folio);
>         }