mm: avoid marking swap cached page as lazyfree

MADV_FREE clears pte dirty bit and then marks the page lazyfree (clear
SwapBacked).  There is no lock to prevent the page is added to swap
cache between these two steps by page reclaim.  Page reclaim could add
the page to swap cache and unmap the page.  After page reclaim, the page
is added back to lru.  At that time, we probably start draining per-cpu
pagevec and mark the page lazyfree.  So the page could be in a state
with SwapBacked cleared and PG_swapcache set.  Next time there is a
refault in the virtual address, do_swap_page can find the page from swap
cache but the page has PageSwapCache false because SwapBacked isn't set,
so do_swap_page will bail out and do nothing.  The task will keep
running into fault handler.

Fixes: 802a3a9 ("mm: reclaim MADV_FREE pages")
Link: http://lkml.kernel.org/r/6537ef3814398c0073630b03f176263bc81f0902.1506446061.git.shli@fb.com
Signed-off-by: Shaohua Li <[email protected]>
Reported-by: Artem Savkov <[email protected]>
Tested-by: Artem Savkov <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Acked-by: Minchan Kim <[email protected]>
Cc: Hillf Danton <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: <[email protected]>	[4.12+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/swap.c

@@ -575,7 +575,7 @@ static void lru_lazyfree_fn(struct page *page, struct lruvec *lruvec,
 			    void *arg)
 {
 	if (PageLRU(page) && PageAnon(page) && PageSwapBacked(page) &&
-	    !PageUnevictable(page)) {
+	    !PageSwapCache(page) && !PageUnevictable(page)) {
 		bool active = PageActive(page);
 
 		del_page_from_lru_list(page, lruvec,
@@ -665,7 +665,7 @@ void deactivate_file_page(struct page *page)
 void mark_page_lazyfree(struct page *page)
 {
 	if (PageLRU(page) && PageAnon(page) && PageSwapBacked(page) &&
-	    !PageUnevictable(page)) {
+	    !PageSwapCache(page) && !PageUnevictable(page)) {
 		struct pagevec *pvec = &get_cpu_var(lru_lazyfree_pvecs);
 
 		get_page(page);