mm, mlock, vmscan: no more skipping pagevecs

When a thread mlocks an address space backed either by file pages which
are currently not present in memory or swapped out anon pages (not in
swapcache), a new page is allocated and added to the local pagevec
(lru_add_pvec), I/O is triggered and the thread then sleeps on the page.
On I/O completion, the thread can wake on a different CPU, the mlock
syscall will then sets the PageMlocked() bit of the page but will not be
able to put that page in unevictable LRU as the page is on the pagevec
of a different CPU.  Even on drain, that page will go to evictable LRU
because the PageMlocked() bit is not checked on pagevec drain.

The page will eventually go to right LRU on reclaim but the LRU stats
will remain skewed for a long time.

This patch puts all the pages, even unevictable, to the pagevecs and on
the drain, the pages will be added on their LRUs correctly by checking
their evictability.  This resolves the mlocked pages on pagevec of other
CPUs issue because when those pagevecs will be drained, the mlocked file
pages will go to unevictable LRU.  Also this makes the race with munlock
easier to resolve because the pagevec drains happen in LRU lock.

However there is still one place which makes a page evictable and does
PageLRU check on that page without LRU lock and needs special attention.
TestClearPageMlocked() and isolate_lru_page() in clear_page_mlock().

	#0: __pagevec_lru_add_fn	#1: clear_page_mlock

	SetPageLRU()			if (!TestClearPageMlocked())
					  return
	smp_mb() // <--required
					// inside does PageLRU
	if (!PageMlocked())		if (isolate_lru_page())
	  move to evictable LRU		  putback_lru_page()
	else
	  move to unevictable LRU

In '#1', TestClearPageMlocked() provides full memory barrier semantics
and thus the PageLRU check (inside isolate_lru_page) can not be
reordered before it.

In '#0', without explicit memory barrier, the PageMlocked() check can be
reordered before SetPageLRU().  If that happens, '#0' can put a page in
unevictable LRU and '#1' might have just cleared the Mlocked bit of that
page but fails to isolate as PageLRU fails as '#0' still hasn't set
PageLRU bit of that page.  That page will be stranded on the unevictable
LRU.

There is one (good) side effect though.  Without this patch, the pages
allocated for System V shared memory segment are added to evictable LRUs
even after shmctl(SHM_LOCK) on that segment.  This patch will correctly
put such pages to unevictable LRU.

Link: http://lkml.kernel.org/r/20171121211241.18877-1-shakeelb@google.com
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Greg Thelen <gthelen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Shaohua Li <shli@fb.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 58 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 444749669187..bee53495a829 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -769,64 +769,7 @@ int remove_mapping(struct address_space *mapping, struct page *page)
  */
 void putback_lru_page(struct page *page)
 {
-	bool is_unevictable;
-	int was_unevictable = PageUnevictable(page);
-
-	VM_BUG_ON_PAGE(PageLRU(page), page);
-
-redo:
-	ClearPageUnevictable(page);
-
-	if (page_evictable(page)) {
-		/*
-		 * For evictable pages, we can use the cache.
-		 * In event of a race, worst case is we end up with an
-		 * unevictable page on [in]active list.
-		 * We know how to handle that.
-		 */
-		is_unevictable = false;
-		lru_cache_add(page);
-	} else {
-		/*
-		 * Put unevictable pages directly on zone's unevictable
-		 * list.
-		 */
-		is_unevictable = true;
-		add_page_to_unevictable_list(page);
-		/*
-		 * When racing with an mlock or AS_UNEVICTABLE clearing
-		 * (page is unlocked) make sure that if the other thread
-		 * does not observe our setting of PG_lru and fails
-		 * isolation/check_move_unevictable_pages,
-		 * we see PG_mlocked/AS_UNEVICTABLE cleared below and move
-		 * the page back to the evictable list.
-		 *
-		 * The other side is TestClearPageMlocked() or shmem_lock().
-		 */
-		smp_mb();
-	}
-
-	/*
-	 * page's status can change while we move it among lru. If an evictable
-	 * page is on unevictable list, it never be freed. To avoid that,
-	 * check after we added it to the list, again.
-	 */
-	if (is_unevictable && page_evictable(page)) {
-		if (!isolate_lru_page(page)) {
-			put_page(page);
-			goto redo;
-		}
-		/* This means someone else dropped this page from LRU
-		 * So, it will be freed or putback to LRU again. There is
-		 * nothing to do here.
-		 */
-	}
-
-	if (was_unevictable && !is_unevictable)
-		count_vm_event(UNEVICTABLE_PGRESCUED);
-	else if (!was_unevictable && is_unevictable)
-		count_vm_event(UNEVICTABLE_PGCULLED);
-
+	lru_cache_add(page);
 	put_page(page);		/* drop ref from isolate */
 }