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() and release_pages() (and the presumably copy-and-pasted put_zone_device_private_or_public_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 wake_up_page_bit(), and delete the superfluous comments. Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> --- We've used this since 2018, and I see Yu Zhao posted similar in 2020: https://lore.kernel.org/linux-mm/20200818184704.3625199-3-yuzhao@xxxxxxxxxx/ I couldn't join in at that time, but think its reception was over-cautious. include/linux/page-flags.h | 2 +- mm/filemap.c | 22 +++++++--------------- mm/memremap.c | 2 -- mm/swap.c | 4 ---- 4 files changed, 8 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 +1179,16 @@ 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. * - * 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. + * That's okay, it's a rare case. The next waker will clear it. Or, + * it might be left set until the page is freed: when it's masked off + * with others in PAGE_FLAGS_CHECK_AT_PREP, by free_pages_prepare(). */ - 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)
