[PATCH 4.5 047/101] mm: thp: calculate the mapcount correctly for THP pages during WP faults

Greg Kroah-Hartman
2016-05-17T01:44:31Z

From: Andrea Arcangeli <aarcange@redhat.com>

commit 6d0a07edd17cfc12fdc1f36de8072fa17cc3666f upstream.

This will provide fully accuracy to the mapcount calculation in the
write protect faults, so page pinning will not get broken by false
positive copy-on-writes.

total_mapcount() isn't the right calculation needed in
reuse_swap_page(), so this introduces a page_trans_huge_mapcount()
that is effectively the full accurate return value for page_mapcount()
if dealing with Transparent Hugepages, however we only use the
page_trans_huge_mapcount() during COW faults where it strictly needed,
due to its higher runtime cost.

This also provide at practical zero cost the total_mapcount
information which is needed to know if we can still relocate the page
anon_vma to the local vma. If page_trans_huge_mapcount() returns 1 we
can reuse the page no matter if it's a pte or a pmd_trans_huge
triggering the fault, but we can only relocate the page anon_vma to
the local vma->anon_vma if we're sure it's only this "vma" mapping the
whole THP physical range.

Kirill A. Shutemov discovered the problem with moving the page
anon_vma to the local vma->anon_vma in a previous version of this
patch and another problem in the way page_move_anon_rmap() was called.

Andrew Morton discovered that CONFIG_SWAP=n wouldn't build in a
previous version, because reuse_swap_page must be a macro to call
page_trans_huge_mapcount from swap.h, so this uses a macro again
instead of an inline function. With this change at least it's a less
dangerous usage than it was before, because "page" is used only once
now, while with the previous code reuse_swap_page(page++) would have
called page_mapcount on page+1 and it would have increased page twice
instead of just once.

Dean Luick noticed an uninitialized variable that could result in a
rmap inefficiency for the non-THP case in a previous version.

Mike Marciniszyn said:

: Our RDMA tests are seeing an issue with memory locking that bisects to
: commit 61f5d698cc97 ("mm: re-enable THP")
:
: The test program registers two rather large MRs (512M) and RDMA
: writes data to a passive peer using the first and RDMA reads it back
: into the second MR and compares that data. The sizes are chosen randomly
: between 0 and 1024 bytes.
:
: The test will get through a few (<= 4 iterations) and then gets a
: compare error.
:
: Tracing indicates the kernel logical addresses associated with the individual
: pages at registration ARE correct , the data in the "RDMA read response only"
: packets ARE correct.
:
: The "corruption" occurs when the packet crosse two pages that are not physically
: contiguous. The second page reads back as zero in the program.
:
: It looks like the user VA at the point of the compare error no longer points to
: the same physical address as was registered.
:
: This patch totally resolves the issue!

Link: http://lkml.kernel.org/r/1462547040-1737-2-git-send-email-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: "Kirill A. Shutemov" <kirill@shutemov.name>
Reviewed-by: Dean Luick <dean.luick@intel.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Tested-by: Josh Collier <josh.d.collier@intel.com>
Cc: Marc Haber <mh+linux-kernel@zugschlus.de>
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>

---
include/linux/mm.h | 9 ++++++
include/linux/swap.h | 6 ++--
mm/huge_memory.c | 71 ++++++++++++++++++++++++++++++++++++++++++++-------
mm/memory.c | 22 ++++++++++-----
mm/swapfile.c | 13 +++++----
5 files changed, 95 insertions(+), 26 deletions(-)

--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -456,11 +456,20 @@ static inline int page_mapcount(struct p

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
int total_mapcount(struct page *page);
+int page_trans_huge_mapcount(struct page *page, int *total_mapcount);
#else
static inline int total_mapcount(struct page *page)
{
return page_mapcount(page);
}
+static inline int page_trans_huge_mapcount(struct page *page,
+ int *total_mapcount)
+{
+ int mapcount = page_mapcount(page);
+ if (total_mapcount)
+ *total_mapcount = mapcount;
+ return mapcount;
+}
#endif

static inline int page_count(struct page *page)
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -418,7 +418,7 @@ extern sector_t swapdev_block(int, pgoff
extern int page_swapcount(struct page *);
extern int swp_swapcount(swp_entry_t entry);
extern struct swap_info_struct *page_swap_info(struct page *);
-extern int reuse_swap_page(struct page *);
+extern bool reuse_swap_page(struct page *, int *);
extern int try_to_free_swap(struct page *);
struct backing_dev_info;

@@ -513,8 +513,8 @@ static inline int swp_swapcount(swp_entr
return 0;
}

-#define reuse_swap_page(page) \
- (!PageTransCompound(page) && page_mapcount(page) == 1)
+#define reuse_swap_page(page, total_mapcount) \
+ (page_trans_huge_mapcount(page, total_mapcount) == 1)

static inline int try_to_free_swap(struct page *page)
{
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1257,15 +1257,9 @@ int do_huge_pmd_wp_page(struct mm_struct
VM_BUG_ON_PAGE(!PageCompound(page) || !PageHead(page), page);
/*
* We can only reuse the page if nobody else maps the huge page or it's
- * part. We can do it by checking page_mapcount() on each sub-page, but
- * it's expensive.
- * The cheaper way is to check page_count() to be equal 1: every
- * mapcount takes page reference reference, so this way we can
- * guarantee, that the PMD is the only mapping.
- * This can give false negative if somebody pinned the page, but that's
- * fine.
+ * part.
*/
- if (page_mapcount(page) == 1 && page_count(page) == 1) {
+ if (page_trans_huge_mapcount(page, NULL) == 1) {
pmd_t entry;
entry = pmd_mkyoung(orig_pmd);
entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
@@ -2038,7 +2032,8 @@ static int __collapse_huge_page_isolate(
if (pte_write(pteval)) {
writable = true;
} else {
- if (PageSwapCache(page) && !reuse_swap_page(page)) {
+ if (PageSwapCache(page) &&
+ !reuse_swap_page(page, NULL)) {
unlock_page(page);
result = SCAN_SWAP_CACHE_PAGE;
goto out;
@@ -3337,6 +3332,64 @@ int total_mapcount(struct page *page)
return ret;
}

+/*
+ * This calculates accurately how many mappings a transparent hugepage
+ * has (unlike page_mapcount() which isn't fully accurate). This full
+ * accuracy is primarily needed to know if copy-on-write faults can
+ * reuse the page and change the mapping to read-write instead of
+ * copying them. At the same time this returns the total_mapcount too.
+ *
+ * The function returns the highest mapcount any one of the subpages
+ * has. If the return value is one, even if different processes are
+ * mapping different subpages of the transparent hugepage, they can
+ * all reuse it, because each process is reusing a different subpage.
+ *
+ * The total_mapcount is instead counting all virtual mappings of the
+ * subpages. If the total_mapcount is equal to "one", it tells the
+ * caller all mappings belong to the same "mm" and in turn the
+ * anon_vma of the transparent hugepage can become the vma->anon_vma
+ * local one as no other process may be mapping any of the subpages.
+ *
+ * It would be more accurate to replace page_mapcount() with
+ * page_trans_huge_mapcount(), however we only use
+ * page_trans_huge_mapcount() in the copy-on-write faults where we
+ * need full accuracy to avoid breaking page pinning, because
+ * page_trans_huge_mapcount() is slower than page_mapcount().
+ */
+int page_trans_huge_mapcount(struct page *page, int *total_mapcount)
+{
+ int i, ret, _total_mapcount, mapcount;
+
+ /* hugetlbfs shouldn't call it */
+ VM_BUG_ON_PAGE(PageHuge(page), page);
+
+ if (likely(!PageTransCompound(page))) {
+ mapcount = atomic_read(&page->_mapcount) + 1;
+ if (total_mapcount)
+ *total_mapcount = mapcount;
+ return mapcount;
+ }
+
+ page = compound_head(page);
+
+ _total_mapcount = ret = 0;
+ for (i = 0; i < HPAGE_PMD_NR; i++) {
+ mapcount = atomic_read(&page[i]._mapcount) + 1;
+ ret = max(ret, mapcount);
+ _total_mapcount += mapcount;
+ }
+ if (PageDoubleMap(page)) {
+ ret -= 1;
+ _total_mapcount -= HPAGE_PMD_NR;
+ }
+ mapcount = compound_mapcount(page);
+ ret += mapcount;
+ _total_mapcount += mapcount;
+ if (total_mapcount)
+ *total_mapcount = _total_mapcount;
+ return ret;
+}
+
/*
* This function splits huge page into normal pages. @page can point to any
* subpage of huge page to split. Split doesn't change the position of @page.
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2357,6 +2357,7 @@ static int do_wp_page(struct mm_struct *
* not dirty accountable.
*/
if (PageAnon(old_page) && !PageKsm(old_page)) {
+ int total_mapcount;
if (!trylock_page(old_page)) {
page_cache_get(old_page);
pte_unmap_unlock(page_table, ptl);
@@ -2371,13 +2372,18 @@ static int do_wp_page(struct mm_struct *
}
page_cache_release(old_page);
}
- if (reuse_swap_page(old_page)) {
- /*
- * The page is all ours. Move it to our anon_vma so
- * the rmap code will not search our parent or siblings.
- * Protected against the rmap code by the page lock.
- */
- page_move_anon_rmap(old_page, vma, address);
+ if (reuse_swap_page(old_page, &total_mapcount)) {
+ if (total_mapcount == 1) {
+ /*
+ * The page is all ours. Move it to
+ * our anon_vma so the rmap code will
+ * not search our parent or siblings.
+ * Protected against the rmap code by
+ * the page lock.
+ */
+ page_move_anon_rmap(compound_head(old_page),
+ vma, address);
+ }
unlock_page(old_page);
return wp_page_reuse(mm, vma, address, page_table, ptl,
orig_pte, old_page, 0, 0);
@@ -2602,7 +2608,7 @@ static int do_swap_page(struct mm_struct
inc_mm_counter_fast(mm, MM_ANONPAGES);
dec_mm_counter_fast(mm, MM_SWAPENTS);
pte = mk_pte(page, vma->vm_page_prot);
- if ((flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) {
+ if ((flags & FAULT_FLAG_WRITE) && reuse_swap_page(page, NULL)) {
pte = maybe_mkwrite(pte_mkdirty(pte), vma);
flags &= ~FAULT_FLAG_WRITE;
ret |= VM_FAULT_WRITE;
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -916,18 +916,19 @@ out:
* to it. And as a side-effect, free up its swap: because the old content
* on disk will never be read, and seeking back there to write new content
* later would only waste time away from clustering.
+ *
+ * NOTE: total_mapcount should not be relied upon by the caller if
+ * reuse_swap_page() returns false, but it may be always overwritten
+ * (see the other implementation for CONFIG_SWAP=n).
*/
-int reuse_swap_page(struct page *page)
+bool reuse_swap_page(struct page *page, int *total_mapcount)
{
int count;

VM_BUG_ON_PAGE(!PageLocked(page), page);
if (unlikely(PageKsm(page)))
- return 0;
- /* The page is part of THP and cannot be reused */
- if (PageTransCompound(page))
- return 0;
- count = page_mapcount(page);
+ return false;
+ count = page_trans_huge_mapcount(page, total_mapcount);
if (count <= 1 && PageSwapCache(page)) {
count += page_swapcount(page);
if (count == 1 && !PageWriteback(page)) {