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Date:      Sun, 3 Feb 2019 18:38:59 +0000 (UTC)
From:      Mark Johnston <markj@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r343719 - head/sys/vm
Message-ID:  <201902031838.x13IcxQN035885@repo.freebsd.org>
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Author: markj
Date: Sun Feb  3 18:38:58 2019
New Revision: 343719
URL: https://svnweb.freebsd.org/changeset/base/343719

Log:
  Fix a race in vm_page_dequeue_deferred().
  
  To detect the case where the page is already marked for a deferred
  dequeue, we must read the "queue" and "aflags" fields in a
  precise order.  Otherwise, a race with a concurrent
  vm_page_dequeue_complete() could leave the page with PGA_DEQUEUE
  set despite it already having been dequeued.  Fix the problem by
  using vm_page_queue() to check the queue state, which correctly
  handles the race.
  
  Reviewed by:	kib
  Tested by:	pho
  MFC after:	3 days
  Sponsored by:	Netflix
  Differential Revision:	https://reviews.freebsd.org/D19039

Modified:
  head/sys/vm/vm_page.c

Modified: head/sys/vm/vm_page.c
==============================================================================
--- head/sys/vm/vm_page.c	Sun Feb  3 15:34:09 2019	(r343718)
+++ head/sys/vm/vm_page.c	Sun Feb  3 18:38:58 2019	(r343719)
@@ -3279,18 +3279,13 @@ vm_page_dequeue_complete(vm_page_t m)
 void
 vm_page_dequeue_deferred(vm_page_t m)
 {
-	int queue;
+	uint8_t queue;
 
 	vm_page_assert_locked(m);
 
-	queue = atomic_load_8(&m->queue);
-	if (queue == PQ_NONE) {
-		KASSERT((m->aflags & PGA_QUEUE_STATE_MASK) == 0,
-		    ("page %p has queue state", m));
+	if ((queue = vm_page_queue(m)) == PQ_NONE)
 		return;
-	}
-	if ((m->aflags & PGA_DEQUEUE) == 0)
-		vm_page_aflag_set(m, PGA_DEQUEUE);
+	vm_page_aflag_set(m, PGA_DEQUEUE);
 	vm_pqbatch_submit_page(m, queue);
 }
 
@@ -3386,7 +3381,7 @@ vm_page_requeue(vm_page_t m)
 {
 
 	vm_page_assert_locked(m);
-	KASSERT(m->queue != PQ_NONE,
+	KASSERT(vm_page_queue(m) != PQ_NONE,
 	    ("%s: page %p is not logically enqueued", __func__, m));
 
 	if ((m->aflags & PGA_REQUEUE) == 0)

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