mm/thp: fix NR_FILE_MAPPED accounting in page_*_file_rmap()

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@google.com
Fixes: dd78fedde4b9 ("rmap: support file thp")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/rmap.c b/mm/rmap.c
index 615b5d3..ee1f10d 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1236,14 +1236,14 @@ void page_add_new_anon_rmap(struct page *page,
 void page_add_file_rmap(struct page *page,
        struct vm_area_struct *vma, 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++;
                }
@@ -1271,11 +1271,12 @@ void page_add_file_rmap(struct page *page,
                        VM_WARN_ON_ONCE(!PageLocked(page));
                        SetPageDoubleMap(compound_head(page));
                }
-               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);
 
        mlock_vma_page(page, vma, compound);
@@ -1283,7 +1284,7 @@ out:
 
 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);
 
@@ -1298,12 +1299,12 @@ 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);
@@ -1311,16 +1312,12 @@ 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);
 }
 
 static void page_remove_anon_compound_rmap(struct page *page)