mm: munlock: fix potential race with THP page split

Since commit ff6a6da60b89 ("mm: accelerate munlock() treatment of THP
pages") munlock skips tail pages of a munlocked THP page.  There is some
attempt to prevent bad consequences of racing with a THP page split, but
code inspection indicates that there are two problems that may lead to a
non-fatal, yet wrong outcome.

First, __split_huge_page_refcount() copies flags including PageMlocked
from the head page to the tail pages.  Clearing PageMlocked by
munlock_vma_page() in the middle of this operation might result in part
of tail pages left with PageMlocked flag.  As the head page still
appears to be a THP page until all tail pages are processed,
munlock_vma_page() might think it munlocked the whole THP page and skip
all the former tail pages.  Before ff6a6da60, those pages would be
cleared in further iterations of munlock_vma_pages_range(), but NR_MLOCK
would still become undercounted (related the next point).

Second, NR_MLOCK accounting is based on call to hpage_nr_pages() after
the PageMlocked is cleared.  The accounting might also become
inconsistent due to race with __split_huge_page_refcount()

- undercount when HUGE_PMD_NR is subtracted, but some tail pages are
  left with PageMlocked set and counted again (only possible before
  ff6a6da60)

- overcount when hpage_nr_pages() sees a normal page (split has already
  finished), but the parallel split has meanwhile cleared PageMlocked from
  additional tail pages

This patch prevents both problems via extending the scope of lru_lock in
munlock_vma_page().  This is convenient because:

- __split_huge_page_refcount() takes lru_lock for its whole operation

- munlock_vma_page() typically takes lru_lock anyway for page isolation

As this becomes a second function where page isolation is done with
lru_lock already held, factor this out to a new
__munlock_isolate_lru_page() function and clean up the code around.

[akpm@linux-foundation.org: avoid a coding-style ugly]
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/mlock.c b/mm/mlock.c
index 10819ed..b30adbe 100644 (file)
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -90,6 +90,26 @@ void mlock_vma_page(struct page *page)
        }
 }
 
+/*
+ * Isolate a page from LRU with optional get_page() pin.
+ * Assumes lru_lock already held and page already pinned.
+ */
+static bool __munlock_isolate_lru_page(struct page *page, bool getpage)
+{
+       if (PageLRU(page)) {
+               struct lruvec *lruvec;
+
+               lruvec = mem_cgroup_page_lruvec(page, page_zone(page));
+               if (getpage)
+                       get_page(page);
+               ClearPageLRU(page);
+               del_page_from_lru_list(page, lruvec, page_lru(page));
+               return true;
+       }
+
+       return false;
+}
+
 /*
  * Finish munlock after successful page isolation
  *
@@ -126,9 +146,9 @@ static void __munlock_isolated_page(struct page *page)
 static void __munlock_isolation_failed(struct page *page)
 {
        if (PageUnevictable(page))
-               count_vm_event(UNEVICTABLE_PGSTRANDED);
+               __count_vm_event(UNEVICTABLE_PGSTRANDED);
        else
-               count_vm_event(UNEVICTABLE_PGMUNLOCKED);
+               __count_vm_event(UNEVICTABLE_PGMUNLOCKED);
 }
 
 /**
@@ -152,28 +172,34 @@ static void __munlock_isolation_failed(struct page *page)
 unsigned int munlock_vma_page(struct page *page)
 {
        unsigned int nr_pages;
+       struct zone *zone = page_zone(page);
 
        BUG_ON(!PageLocked(page));
 
-       if (TestClearPageMlocked(page)) {
-               nr_pages = hpage_nr_pages(page);
-               mod_zone_page_state(page_zone(page), NR_MLOCK, -nr_pages);
-               if (!isolate_lru_page(page))
-                       __munlock_isolated_page(page);
-               else
-                       __munlock_isolation_failed(page);
-       } else {
-               nr_pages = hpage_nr_pages(page);
-       }
-
        /*
-        * Regardless of the original PageMlocked flag, we determine nr_pages
-        * after touching the flag. This leaves a possible race with a THP page
-        * split, such that a whole THP page was munlocked, but nr_pages == 1.
-        * Returning a smaller mask due to that is OK, the worst that can
-        * happen is subsequent useless scanning of the former tail pages.
-        * The NR_MLOCK accounting can however become broken.
+        * Serialize with any parallel __split_huge_page_refcount() which
+        * might otherwise copy PageMlocked to part of the tail pages before
+        * we clear it in the head page. It also stabilizes hpage_nr_pages().
         */
+       spin_lock_irq(&zone->lru_lock);
+
+       nr_pages = hpage_nr_pages(page);
+       if (!TestClearPageMlocked(page))
+               goto unlock_out;
+
+       __mod_zone_page_state(zone, NR_MLOCK, -nr_pages);
+
+       if (__munlock_isolate_lru_page(page, true)) {
+               spin_unlock_irq(&zone->lru_lock);
+               __munlock_isolated_page(page);
+               goto out;
+       }
+       __munlock_isolation_failed(page);
+
+unlock_out:
+       spin_unlock_irq(&zone->lru_lock);
+
+out:
        return nr_pages - 1;
 }
 
@@ -310,34 +336,24 @@ static void __munlock_pagevec(struct pagevec *pvec, struct zone *zone)
                struct page *page = pvec->pages[i];
 
                if (TestClearPageMlocked(page)) {
-                       struct lruvec *lruvec;
-                       int lru;
-
-                       if (PageLRU(page)) {
-                               lruvec = mem_cgroup_page_lruvec(page, zone);
-                               lru = page_lru(page);
-                               /*
-                                * We already have pin from follow_page_mask()
-                                * so we can spare the get_page() here.
-                                */
-                               ClearPageLRU(page);
-                               del_page_from_lru_list(page, lruvec, lru);
-                       } else {
-                               __munlock_isolation_failed(page);
-                               goto skip_munlock;
-                       }
-
-               } else {
-skip_munlock:
                        /*
-                        * We won't be munlocking this page in the next phase
-                        * but we still need to release the follow_page_mask()
-                        * pin. We cannot do it under lru_lock however. If it's
-                        * the last pin, __page_cache_release would deadlock.
+                        * We already have pin from follow_page_mask()
+                        * so we can spare the get_page() here.
                         */
-                       pagevec_add(&pvec_putback, pvec->pages[i]);
-                       pvec->pages[i] = NULL;
+                       if (__munlock_isolate_lru_page(page, false))
+                               continue;
+                       else
+                               __munlock_isolation_failed(page);
                }
+
+               /*
+                * We won't be munlocking this page in the next phase
+                * but we still need to release the follow_page_mask()
+                * pin. We cannot do it under lru_lock however. If it's
+                * the last pin, __page_cache_release() would deadlock.
+                */
+               pagevec_add(&pvec_putback, pvec->pages[i]);
+               pvec->pages[i] = NULL;
        }
        delta_munlocked = -nr + pagevec_count(&pvec_putback);
        __mod_zone_page_state(zone, NR_MLOCK, delta_munlocked);