Re: [PATCH v3] mm/rmap: do not add fully unmapped large folio to deferred split list

[Lance Yang]

Hey Zi,

On Thu, Apr 25, 2024 at 6:46 AM Zi Yan <zi.yan@xxxxxxxx> wrote:
>
> From: Zi Yan <ziy@xxxxxxxxxx>
>
> In __folio_remove_rmap(), a large folio is added to deferred split list
> if any page in a folio loses its final mapping. It is possible that
> the folio is unmapped fully, but it is unnecessary to add the folio

Agreed. If a folio is fully unmapped, then that's unnecessary to add
to the deferred split list.

> to deferred split list at all. Fix it by checking folio->_nr_pages_mapped
> before adding a folio to deferred split list. If the folio is already
> on the deferred split list, it will be skipped. This issue applies to
> both PTE-mapped THP and mTHP.
>
> Commit 98046944a159 ("mm: huge_memory: add the missing
> folio_test_pmd_mappable() for THP split statistics") tried to exclude
> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not
> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still
> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,
> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside
> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable().
> However, this miscount was present even earlier due to implementation,
> since PTEs are unmapped individually and first PTE unmapping adds the THP
> into the deferred split list.
>
> With commit b06dc281aa99 ("mm/rmap: introduce
> folio_remove_rmap_[pte|ptes|pmd]()"), kernel is able to unmap PTE-mapped
> folios in one shot without causing the miscount, hence this patch.
>
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
> ---
>  mm/rmap.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index a7913a454028..2809348add7b 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1553,9 +1553,10 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>                  * page of the folio is unmapped and at least one page
>                  * is still mapped.
>                  */
> -               if (folio_test_large(folio) && folio_test_anon(folio))
> -                       if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
> -                               deferred_split_folio(folio);
> +               if (folio_test_large(folio) && folio_test_anon(folio) &&
> +                   ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
> +                    (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))

Perhaps we only need to check the mapcount?

IIUC, if a large folio that was PMD/PTE mapped is fully unmapped here,
then folio_mapcount() will return 0.

-               if (folio_test_large(folio) && folio_test_anon(folio))
-                       if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
-                               deferred_split_folio(folio);
+               if (folio_test_large(folio) && folio_test_anon(folio) &&
+                   folio_mapcount(folio))
+                       deferred_split_folio(folio);

Thanks,
Lance




> +                       deferred_split_folio(folio);
>         }
>
>         /*
>
> base-commit: 2541ee5668b019c486dd3e815114130e35c1495d
> --
> 2.43.0
>