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

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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>
This commit is contained in:
Hugh Dickins 2022-03-24 18:09:55 -07:00 committed by Linus Torvalds
parent 85207ad8ea
commit 5d543f13e2
1 changed files with 14 additions and 17 deletions
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31
mm/rmap.c
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@ -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)