The patch titled
Subject: mm/thp: fix NR_FILE_MAPPED accounting in page_*_file_rmap()
has been added to the -mm tree. Its filename is
mm-thp-fix-nr_file_mapped-accounting-in-page__file_rmap.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-thp-fix-nr_file_mapped-accounting-in-page__file_rmap.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-thp-fix-nr_file_mapped-accounting-in-page__file_rmap.patch
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------------------------------------------------------
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)
_
Patches currently in -mm which might be from hughd@xxxxxxxxxx are
memfd-fix-f_seal_write-after-shmem-huge-page-allocated.patch
tmpfs-support-for-file-creation-time-fix.patch
mm-thp-clearpagedoublemap-in-first-page_add_file_rmap.patch
mm-thp-fix-nr_file_mapped-accounting-in-page__file_rmap.patch
