Commit 5d543f13 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
Browse files

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: dd78fedd ("rmap: support file thp")
Signed-off-by: default avatarHugh Dickins <hughd@google.com>
Reviewed-by: default avatarYang Shi <shy828301@gmail.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 85207ad8

mm/rmap.c

+14 −17
Original line number Diff line number Diff line
@@ -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 @@ void page_add_file_rmap(struct page *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);
@@ -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);
@@ -1311,15 +1312,11 @@ 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.

	 */

out:

	if (nr)

		__mod_lruvec_page_state(page, NR_FILE_MAPPED, -nr);

}