[PATCH mmotm] mm: delete __ClearPageWaiters()

[Hugh Dickins <hughd@google.com>]

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@google.com>
---
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@google.com/
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)