From a449cccb80e0c19c72339e213f14d6ca97df4eb0 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sun, 27 Aug 2017 16:25:09 -0700
Subject: [PATCH] Avoid page waitqueue race leaving possible page locker
 waiting

References: bnc#971975 VM performance -- page waitqueue
Patch-mainline: v4.13-rc7
Git-commit: a8b169afbf06a678437632709caac98e16f99263

The "lock_page_killable()" function waits for exclusive access to the
page lock bit using the WQ_FLAG_EXCLUSIVE bit in the waitqueue entry
set.

That means that if it gets woken up, other waiters may have been
skipped.

That, in turn, means that if it sees the page being unlocked, it *must*
take that lock and return success, even if a lethal signal is also
pending.

So instead of checking for lethal signals first, we need to check for
them after we've checked the actual bit that we were waiting for.  Even
if that might then delay the killing of the process.

This matches the order of the old "wait_on_bit_lock()" infrastructure
that the page locking used to use (and is still used in a few other
areas).

Note that if we still return an error after having unsuccessfully tried
to acquire the page lock, that is ok: that means that some other thread
was able to get ahead of us and lock the page, and when that other
thread then unlocks the page, the wakeup event will be repeated.  So any
other pending waiters will now get properly woken up.

Fixes: 62906027091f ("mm: add PageWaiters indicating tasks are waiting for a page bit")
Cc: Nick Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Jan Kara <jack@suse.cz>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 mm/filemap.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 1b37ca88ccac..6a6e0bd1b541 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -892,10 +892,6 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
 
 		if (likely(test_bit(bit_nr, &page->flags))) {
 			io_schedule();
-			if (unlikely(signal_pending_state(state, current))) {
-				ret = -EINTR;
-				break;
-			}
 		}
 
 		if (lock) {
@@ -905,6 +901,11 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
 			if (!test_bit(bit_nr, &page->flags))
 				break;
 		}
+
+		if (unlikely(signal_pending_state(state, current))) {
+			ret = -EINTR;
+			break;
+		}
 	}
 
 	finish_wait(q, wait);