Re: [PATCH mmotm v2] mm: delete __ClearPageWaiters()

[Yang Shi <shy828301@gmail.com>]

On Fri, Mar 4, 2022 at 1:25 PM Hugh Dickins <hughd@google.com> wrote:
>
> The PG_waiters bit is not included in PAGE_FLAGS_CHECK_AT_FREE, and
> vmscan.c's free_unref_page_list() callers rely on that not to generate
> bad_page() alerts.  So __page_cache_release(), put_pages_list() and
> release_pages() (and presumably copy-and-pasted free_zone_device_page())
> are redundant and misleading to make a special point of clearing it (as
> the "__" implies, it could only safely be used on the freeing path).
>
> Delete __ClearPageWaiters().  Remark on this in one of the "possible"
> comments in folio_wake_bit(), and delete the superfluous comments.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Tested-by: Yu Zhao <yuzhao@google.com>
> ---
> v2: Add Tested-by from Yu Zhao.
>     Update to latest function names in the commit message.
>     Reword comment in folio_wake_bit() per David Hildenbrand.

Reviewed-by: Yang Shi <shy828301@gmail.com>

>
>  include/linux/page-flags.h |  2 +-
>  mm/filemap.c               | 23 ++++++++---------------
>  mm/memremap.c              |  2 --
>  mm/swap.c                  |  4 ----
>  4 files changed, 9 insertions(+), 22 deletions(-)
>
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -481,7 +481,7 @@ static unsigned long *folio_flags(struct folio *folio, unsigned n)
>         TESTSETFLAG_FALSE(uname, lname) TESTCLEARFLAG_FALSE(uname, lname)
>
>  __PAGEFLAG(Locked, locked, PF_NO_TAIL)
> -PAGEFLAG(Waiters, waiters, PF_ONLY_HEAD) __CLEARPAGEFLAG(Waiters, waiters, PF_ONLY_HEAD)
> +PAGEFLAG(Waiters, waiters, PF_ONLY_HEAD)
>  PAGEFLAG(Error, error, PF_NO_TAIL) TESTCLEARFLAG(Error, error, PF_NO_TAIL)
>  PAGEFLAG(Referenced, referenced, PF_HEAD)
>         TESTCLEARFLAG(Referenced, referenced, PF_HEAD)
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1179,24 +1179,17 @@ static void folio_wake_bit(struct folio *folio, int bit_nr)
>         }
>
>         /*
> -        * It is possible for other pages to have collided on the waitqueue
> -        * hash, so in that case check for a page match. That prevents a long-
> -        * term waiter
> +        * It's possible to miss clearing waiters here, when we woke our page
> +        * waiters, but the hashed waitqueue has waiters for other pages on it.
> +        * That's okay, it's a rare case. The next waker will clear it.
>          *
> -        * It is still possible to miss a case here, when we woke page waiters
> -        * and removed them from the waitqueue, but there are still other
> -        * page waiters.
> +        * Note that, depending on the page pool (buddy, hugetlb, ZONE_DEVICE,
> +        * other), the flag may be cleared in the course of freeing the page;
> +        * but that is not required for correctness.
>          */
> -       if (!waitqueue_active(q) || !key.page_match) {
> +       if (!waitqueue_active(q) || !key.page_match)
>                 folio_clear_waiters(folio);
> -               /*
> -                * It's possible to miss clearing Waiters here, when we woke
> -                * our page waiters, but the hashed waitqueue has waiters for
> -                * other pages on it.
> -                *
> -                * That's okay, it's a rare case. The next waker will clear it.
> -                */
> -       }
> +
>         spin_unlock_irqrestore(&q->lock, flags);
>  }
>
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -487,8 +487,6 @@ void free_zone_device_page(struct page *page)
>         if (WARN_ON_ONCE(!page->pgmap->ops || !page->pgmap->ops->page_free))
>                 return;
>
> -       __ClearPageWaiters(page);
> -
>         mem_cgroup_uncharge(page_folio(page));
>
>         /*
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -97,7 +97,6 @@ static void __page_cache_release(struct page *page)
>                 mod_zone_page_state(page_zone(page), NR_MLOCK, -nr_pages);
>                 count_vm_events(UNEVICTABLE_PGCLEARED, nr_pages);
>         }
> -       __ClearPageWaiters(page);
>  }
>
>  static void __put_single_page(struct page *page)
> @@ -152,7 +151,6 @@ void put_pages_list(struct list_head *pages)
>                         continue;
>                 }
>                 /* Cannot be PageLRU because it's passed to us using the lru */
> -               __ClearPageWaiters(page);
>         }
>
>         free_unref_page_list(pages);
> @@ -966,8 +964,6 @@ void release_pages(struct page **pages, int nr)
>                         count_vm_event(UNEVICTABLE_PGCLEARED);
>                 }
>
> -               __ClearPageWaiters(page);
> -
>                 list_add(&page->lru, &pages_to_free);
>         }
>         if (lruvec)