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.
Reviewed-by: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx>
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
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 --git a/include/linux/mm.h b/include/linux/mm.h
index a55e5be..263f229 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -500,11 +500,20 @@ static inline int page_mapcount(struct page *page)
#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 --git a/include/linux/swap.h b/include/linux/swap.h
index 2b83359..acef20d 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -418,7 +418,7 @@ extern sector_t swapdev_block(int, pgoff_t);
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_entry_t entry)
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 --git a/mm/huge_memory.c b/mm/huge_memory.c
index 86f9f8b..9086793 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1298,15 +1298,9 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
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);
@@ -2080,7 +2074,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
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;
@@ -3225,6 +3220,60 @@ int total_mapcount(struct page *page)
}
/*
+ * 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 --git a/mm/memory.c b/mm/memory.c
index 93897f2..d7a2af3 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2340,6 +2340,7 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
* 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);
@@ -2354,13 +2355,18 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
}
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);
@@ -2584,7 +2590,7 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
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 --git a/mm/swapfile.c b/mm/swapfile.c
index 83874ec..031713ab 100644
--- a/mm/swapfile.c
+++ b/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)) {
--
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