The patch titled
Subject: mm: thp: calculate the mapcount correctly for THP pages during WP faults
has been added to the -mm tree. Its filename is
mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults.patch
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------------------------------------------------------
From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Subject: mm: thp: calculate the mapcount correctly for THP pages during WP faults
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.
Link: http://lkml.kernel.org/r/1462547040-1737-2-git-send-email-aarcange@xxxxxxxxxx
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Reviewed-by: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx>
Tested-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [4.5]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mm.h | 9 +++++
include/linux/swap.h | 8 +++-
mm/huge_memory.c | 67 +++++++++++++++++++++++++++++++++++------
mm/memory.c | 22 ++++++++-----
mm/swapfile.c | 13 ++++---
5 files changed, 93 insertions(+), 26 deletions(-)
diff -puN include/linux/mm.h~mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults include/linux/mm.h
--- a/include/linux/mm.h~mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults
+++ a/include/linux/mm.h
@@ -500,11 +500,20 @@ 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 struct page *virt_to_head_page(const void *x)
diff -puN include/linux/swap.h~mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults include/linux/swap.h
--- a/include/linux/swap.h~mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults
+++ a/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 +513,10 @@ static inline int swp_swapcount(swp_entr
return 0;
}
-#define reuse_swap_page(page) \
- (!PageTransCompound(page) && page_mapcount(page) == 1)
+static inline bool reuse_swap_page(struct page *page, int *total_mapcount)
+{
+ return page_trans_huge_mapcount(page, total_mapcount) == 1;
+}
static inline int try_to_free_swap(struct page *page)
{
diff -puN mm/huge_memory.c~mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults mm/huge_memory.c
--- a/mm/huge_memory.c~mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults
+++ a/mm/huge_memory.c
@@ -1298,15 +1298,9 @@ 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);
@@ -2079,7 +2073,8 @@ 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;
@@ -3222,6 +3217,60 @@ 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)))
+ return atomic_read(&page->_mapcount) + 1;
+
+ 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.
diff -puN mm/memory.c~mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults mm/memory.c
--- a/mm/memory.c~mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults
+++ a/mm/memory.c
@@ -2373,6 +2373,7 @@ 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)) {
get_page(old_page);
pte_unmap_unlock(page_table, ptl);
@@ -2387,13 +2388,18 @@ static int do_wp_page(struct mm_struct *
}
put_page(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);
@@ -2617,7 +2623,7 @@ 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;
diff -puN mm/swapfile.c~mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults mm/swapfile.c
--- a/mm/swapfile.c~mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults
+++ a/mm/swapfile.c
@@ -922,18 +922,19 @@ 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)) {
_
Patches currently in -mm which might be from aarcange@xxxxxxxxxx are
mm-thp-calculate-the-mapcount-correctly-for-thp-pages-during-wp-faults.patch
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