From: Hugh Dickins <hughd@xxxxxxxxxx> Subject: 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@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> --- mm/rmap.c | 31 ++++++++++++++----------------- 1 file changed, 14 insertions(+), 17 deletions(-) --- a/mm/rmap.c~mm-thp-fix-nr_file_mapped-accounting-in-page__file_rmap +++ a/mm/rmap.c @@ -1236,14 +1236,14 @@ void page_add_new_anon_rmap(struct 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 *pag 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 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 __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) _
