From: Hugh Dickins hughd@google.com
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@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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- 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);