[PATCH AUTOSEL 14/14] mm/thp: fix NR_FILE_MAPPED accounting in page_*_file_rmap()

[Greg Kroah-Hartman]

From: Hugh Dickins <hughd@xxxxxxxxxx>

commit 5d543f13e2f5580828de885c751d68a35b6a493d upstream.

NR_FILE_MAPPED accounting in mm/rmap.c (for /proc/meminfo "Mapped" and
/proc/vmstat "nr_mapped" and the memcg's memory.stat "mapped_file") is
slightly flawed for file or shmem huge pages.

It is well thought out, and looks convincing, but there's a racy case when
the careful counting in page_remove_file_rmap() (without page lock) gets
discarded.  So that in a workload like two "make -j20" kernel builds under
memory pressure, with cc1 on hugepage text, "Mapped" can easily grow by a
spurious 5MB or more on each iteration, ending up implausibly bigger than
most other numbers in /proc/meminfo.  And, hypothetically, might grow to
the point of seriously interfering in mm/vmscan.c's heuristics, which do
take NR_FILE_MAPPED into some consideration.

Fixed by moving the __mod_lruvec_page_state() down to where it will not be
missed before return (and I've grown a bit tired of that oft-repeated
but-not-everywhere comment on the __ness: it gets lost in the move here).

Does page_add_file_rmap() need the same change?  I suspect not, because
page lock is held in all relevant cases, and its skipping case looks safe;
but it's much easier to be sure, if we do make the same change.

Link: https://lkml.kernel.org/r/e02e52a1-8550-a57c-ed29-f51191ea2375@xxxxxxxxxx
Fixes: dd78fedde4b9 ("rmap: support file thp")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 mm/rmap.c | 30 ++++++++++++++----------------
 1 file changed, 14 insertions(+), 16 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 444d0d958aff..fa09b5eaff34 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1239,14 +1239,14 @@ void page_add_new_anon_rmap(struct page *page,
  */
 void page_add_file_rmap(struct page *page, bool compound)
 {
-	int i, nr = 1;
+	int i, nr = 0;
 
 	VM_BUG_ON_PAGE(compound && !PageTransHuge(page), page);
 	lock_page_memcg(page);
 	if (compound && PageTransHuge(page)) {
 		int nr_pages = thp_nr_pages(page);
 
-		for (i = 0, nr = 0; i < nr_pages; i++) {
+		for (i = 0; i < nr_pages; i++) {
 			if (atomic_inc_and_test(&page[i]._mapcount))
 				nr++;
 		}
@@ -1279,17 +1279,18 @@ void page_add_file_rmap(struct page *page, bool compound)
 			if (PageMlocked(page))
 				clear_page_mlock(head);
 		}
-		if (!atomic_inc_and_test(&page->_mapcount))
-			goto out;
+		if (atomic_inc_and_test(&page->_mapcount))
+			nr++;
 	}
-	__mod_lruvec_page_state(page, NR_FILE_MAPPED, nr);
 out:
+	if (nr)
+		__mod_lruvec_page_state(page, NR_FILE_MAPPED, nr);
 	unlock_page_memcg(page);
 }
 
 static void page_remove_file_rmap(struct page *page, bool compound)
 {
-	int i, nr = 1;
+	int i, nr = 0;
 
 	VM_BUG_ON_PAGE(compound && !PageHead(page), page);
 
@@ -1304,12 +1305,12 @@ static void page_remove_file_rmap(struct page *page, bool compound)
 	if (compound && PageTransHuge(page)) {
 		int nr_pages = thp_nr_pages(page);
 
-		for (i = 0, nr = 0; i < nr_pages; i++) {
+		for (i = 0; i < nr_pages; i++) {
 			if (atomic_add_negative(-1, &page[i]._mapcount))
 				nr++;
 		}
 		if (!atomic_add_negative(-1, compound_mapcount_ptr(page)))
-			return;
+			goto out;
 		if (PageSwapBacked(page))
 			__mod_lruvec_page_state(page, NR_SHMEM_PMDMAPPED,
 						-nr_pages);
@@ -1317,16 +1318,13 @@ static void page_remove_file_rmap(struct page *page, bool compound)
 			__mod_lruvec_page_state(page, NR_FILE_PMDMAPPED,
 						-nr_pages);
 	} else {
-		if (!atomic_add_negative(-1, &page->_mapcount))
-			return;
+		if (atomic_add_negative(-1, &page->_mapcount))
+			nr++;
 	}
 
-	/*
-	 * We use the irq-unsafe __{inc|mod}_lruvec_page_state because
-	 * these counters are not modified in interrupt context, and
-	 * pte lock(a spinlock) is held, which implies preemption disabled.
-	 */
-	__mod_lruvec_page_state(page, NR_FILE_MAPPED, -nr);
+out:
+	if (nr)
+		__mod_lruvec_page_state(page, NR_FILE_MAPPED, -nr);
 
 	if (unlikely(PageMlocked(page)))
 		clear_page_mlock(page);
-- 
2.36.0