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Subject[PATCHv4 14/24] thp: implement new split_huge_page()
Date
The new split_huge_page() can fail if the compound is pinned: we expect
only caller to have one reference to head page. If the page is pinned
split_huge_page() returns -EBUSY and caller must handle this correctly.

We don't need mark PMDs splitting since now we can split one PMD a time
with split_huge_pmd().

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
include/linux/hugetlb_inline.h | 9 +-
include/linux/mm.h | 22 +++--
mm/huge_memory.c | 183 +++++++++++++++++++++++------------------
mm/swap.c | 126 +++++++++++++++++++++++++++-
4 files changed, 244 insertions(+), 96 deletions(-)

diff --git a/include/linux/hugetlb_inline.h b/include/linux/hugetlb_inline.h
index 2bb681fbeb35..c5cd37479731 100644
--- a/include/linux/hugetlb_inline.h
+++ b/include/linux/hugetlb_inline.h
@@ -10,6 +10,8 @@ static inline int is_vm_hugetlb_page(struct vm_area_struct *vma)
return !!(vma->vm_flags & VM_HUGETLB);
}

+int PageHeadHuge(struct page *page_head);
+
#else

static inline int is_vm_hugetlb_page(struct vm_area_struct *vma)
@@ -17,6 +19,11 @@ static inline int is_vm_hugetlb_page(struct vm_area_struct *vma)
return 0;
}

-#endif
+static inline int PageHeadHuge(struct page *page_head)
+{
+ return 0;
+}
+
+#endif /* CONFIG_HUGETLB_PAGE */

#endif
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 020dbbe1563c..28aeae6e553b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -485,20 +485,18 @@ static inline int page_count(struct page *page)
return atomic_read(&compound_head(page)->_count);
}

-#ifdef CONFIG_HUGETLB_PAGE
-extern int PageHeadHuge(struct page *page_head);
-#else /* CONFIG_HUGETLB_PAGE */
-static inline int PageHeadHuge(struct page *page_head)
-{
- return 0;
-}
-#endif /* CONFIG_HUGETLB_PAGE */
-
+void __get_page_tail(struct page *page);
static inline void get_page(struct page *page)
{
- struct page *page_head = compound_head(page);
- VM_BUG_ON_PAGE(atomic_read(&page_head->_count) <= 0, page);
- atomic_inc(&page_head->_count);
+ if (unlikely(PageTail(page)))
+ return __get_page_tail(page);
+
+ /*
+ * Getting a normal page or the head of a compound page
+ * requires to already have an elevated page->_count.
+ */
+ VM_BUG_ON_PAGE(atomic_read(&page->_count) <= 0, page);
+ atomic_inc(&page->_count);
}

static inline struct page *virt_to_head_page(const void *x)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 3409a5c7dbb8..6f6429426edb 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1737,31 +1737,52 @@ static void split_huge_pmd_address(struct vm_area_struct *vma,
__split_huge_pmd(vma, pmd, address);
}

-#if 0
-static void __split_huge_page_refcount(struct page *page,
+static int __split_huge_page_refcount(struct page *page,
struct list_head *list)
{
int i;
struct zone *zone = page_zone(page);
struct lruvec *lruvec;
- int tail_count = 0;
+ int tail_mapcount = 0;

/* prevent PageLRU to go away from under us, and freeze lru stats */
spin_lock_irq(&zone->lru_lock);
lruvec = mem_cgroup_page_lruvec(page, zone);

compound_lock(page);
+
+ /*
+ * We cannot split pinned THP page: we expect page count to be equal
+ * to sum of mapcount of all sub-pages plus one (split_huge_page()
+ * caller must take reference for head page).
+ *
+ * Compound lock only prevents page->_count to be updated from
+ * get_page() or put_page() on tail page. It means means page_count()
+ * can change under us from head page after the check, but it's okay:
+ * all new refernces will stay on head page after split.
+ */
+ tail_mapcount = 0;
+ for (i = 0; i < HPAGE_PMD_NR; i++)
+ tail_mapcount += page_mapcount(page + i);
+ if (tail_mapcount != page_count(page) - 1) {
+ BUG_ON(tail_mapcount > page_count(page) - 1);
+ compound_unlock(page);
+ spin_unlock_irq(&zone->lru_lock);
+ return -EBUSY;
+ }
+
/* complete memcg works before add pages to LRU */
mem_cgroup_split_huge_fixup(page);

+ tail_mapcount = 0;
for (i = HPAGE_PMD_NR - 1; i >= 1; i--) {
struct page *page_tail = page + i;

/* tail_page->_mapcount cannot change */
- BUG_ON(atomic_read(&page_tail->_mapcount) + 1 < 0);
- tail_count += atomic_read(&page_tail->_mapcount) + 1;
+ BUG_ON(page_mapcount(page_tail) < 0);
+ tail_mapcount += page_mapcount(page_tail);
/* check for overflow */
- BUG_ON(tail_count < 0);
+ BUG_ON(tail_mapcount < 0);
BUG_ON(atomic_read(&page_tail->_count) != 0);
/*
* tail_page->_count is zero and not changing from
@@ -1799,28 +1820,9 @@ static void __split_huge_page_refcount(struct page *page,
/* clear PageTail before overwriting first_page */
smp_wmb();

- /*
- * __split_huge_page_splitting() already set the
- * splitting bit in all pmd that could map this
- * hugepage, that will ensure no CPU can alter the
- * mapcount on the head page. The mapcount is only
- * accounted in the head page and it has to be
- * transferred to all tail pages in the below code. So
- * for this code to be safe, the split the mapcount
- * can't change. But that doesn't mean userland can't
- * keep changing and reading the page contents while
- * we transfer the mapcount, so the pmd splitting
- * status is achieved setting a reserved bit in the
- * pmd, not by clearing the present bit.
- */
- atomic_set(&page_tail->_mapcount, compound_mapcount(page) - 1);
-
/* ->mapping in first tail page is compound_mapcount */
- if (i != 1) {
- BUG_ON(page_tail->mapping);
- page_tail->mapping = page->mapping;
- BUG_ON(!PageAnon(page_tail));
- }
+ BUG_ON(i != 1 && page_tail->mapping);
+ page_tail->mapping = page->mapping;

page_tail->index = page->index + i;
page_cpupid_xchg_last(page_tail, page_cpupid_last(page));
@@ -1831,12 +1833,9 @@ static void __split_huge_page_refcount(struct page *page,

lru_add_page_tail(page, page_tail, lruvec, list);
}
- atomic_sub(tail_count, &page->_count);
+ atomic_sub(tail_mapcount, &page->_count);
BUG_ON(atomic_read(&page->_count) <= 0);

- page->_mapcount = *compound_mapcount_ptr(page);
- page[1].mapping = page->mapping;
-
__mod_zone_page_state(zone, NR_ANON_TRANSPARENT_HUGEPAGES, -1);

ClearPageCompound(page);
@@ -1861,71 +1860,95 @@ static void __split_huge_page_refcount(struct page *page,
* to be pinned by the caller.
*/
BUG_ON(page_count(page) <= 0);
+ return 0;
}

-/* must be called with anon_vma->root->rwsem held */
-static void __split_huge_page(struct page *page,
- struct anon_vma *anon_vma,
- struct list_head *list)
+/*
+ * Split a hugepage into normal pages. This doesn't change the position of head
+ * page. If @list is null, tail pages will be added to LRU list, otherwise, to
+ * @list. Both head page and tail pages will inherit mapping, flags, and so on
+ * from the hugepage.
+ * Return 0 if the hugepage is split successfully otherwise return -errno.
+ */
+int split_huge_page_to_list(struct page *page, struct list_head *list)
{
- int mapcount, mapcount2;
- pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
+ struct anon_vma *anon_vma;
struct anon_vma_chain *avc;
+ pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
+ int i, tail_mapcount;
+ int ret = -EBUSY;

- BUG_ON(!PageHead(page));
- BUG_ON(PageTail(page));
+ BUG_ON(is_huge_zero_page(page));
+ BUG_ON(!PageAnon(page));

- mapcount = 0;
- anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
- struct vm_area_struct *vma = avc->vma;
- unsigned long addr = vma_address(page, vma);
- BUG_ON(is_vma_temporary_stack(vma));
- mapcount += __split_huge_page_splitting(page, vma, addr);
- }
/*
- * It is critical that new vmas are added to the tail of the
- * anon_vma list. This guarantes that if copy_huge_pmd() runs
- * and establishes a child pmd before
- * __split_huge_page_splitting() freezes the parent pmd (so if
- * we fail to prevent copy_huge_pmd() from running until the
- * whole __split_huge_page() is complete), we will still see
- * the newly established pmd of the child later during the
- * walk, to be able to set it as pmd_trans_splitting too.
+ * The caller does not necessarily hold an mmap_sem that would prevent
+ * the anon_vma disappearing so we first we take a reference to it
+ * and then lock the anon_vma for write. This is similar to
+ * page_lock_anon_vma_read except the write lock is taken to serialise
+ * against parallel split or collapse operations.
*/
- if (mapcount != page_mapcount(page)) {
- pr_err("mapcount %d page_mapcount %d\n",
- mapcount, page_mapcount(page));
- BUG();
+ anon_vma = page_get_anon_vma(page);
+ if (!anon_vma)
+ goto out;
+ anon_vma_lock_write(anon_vma);
+
+ if (!PageCompound(page)) {
+ ret = 0;
+ goto out_unlock;
}

- __split_huge_page_refcount(page, list);
+ BUG_ON(!PageSwapBacked(page));
+
+ /*
+ * Racy check if __split_huge_page_refcount() can be successful, before
+ * splitting PMDs.
+ */
+ tail_mapcount = compound_mapcount(page);
+ for (i = 0; i < HPAGE_PMD_NR; i++)
+ tail_mapcount += atomic_read(&page[i]._mapcount) + 1;
+ if (tail_mapcount != page_count(page) - 1) {
+ VM_BUG_ON_PAGE(tail_mapcount > page_count(page) - 1, page);
+ ret = -EBUSY;
+ goto out_unlock;
+ }

- mapcount2 = 0;
anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff, pgoff) {
struct vm_area_struct *vma = avc->vma;
unsigned long addr = vma_address(page, vma);
- BUG_ON(is_vma_temporary_stack(vma));
- mapcount2 += __split_huge_page_map(page, vma, addr);
- }
- if (mapcount != mapcount2) {
- pr_err("mapcount %d mapcount2 %d page_mapcount %d\n",
- mapcount, mapcount2, page_mapcount(page));
- BUG();
+ spinlock_t *ptl;
+ pmd_t *pmd;
+ unsigned long haddr = addr & HPAGE_PMD_MASK;
+ unsigned long mmun_start; /* For mmu_notifiers */
+ unsigned long mmun_end; /* For mmu_notifiers */
+
+ mmun_start = haddr;
+ mmun_end = haddr + HPAGE_PMD_SIZE;
+ mmu_notifier_invalidate_range_start(vma->vm_mm,
+ mmun_start, mmun_end);
+ pmd = page_check_address_pmd(page, vma->vm_mm, addr,
+ PAGE_CHECK_ADDRESS_PMD_FLAG, &ptl);
+ if (pmd) {
+ __split_huge_pmd_locked(vma, pmd, addr);
+ spin_unlock(ptl);
+ }
+ mmu_notifier_invalidate_range_end(vma->vm_mm,
+ mmun_start, mmun_end);
}
-}
-#endif

-/*
- * Split a hugepage into normal pages. This doesn't change the position of head
- * page. If @list is null, tail pages will be added to LRU list, otherwise, to
- * @list. Both head page and tail pages will inherit mapping, flags, and so on
- * from the hugepage.
- * Return 0 if the hugepage is split successfully otherwise return -errno.
- */
-int split_huge_page_to_list(struct page *page, struct list_head *list)
-{
- count_vm_event(THP_SPLIT_PAGE_FAILED);
- return -EBUSY;
+ BUG_ON(compound_mapcount(page));
+ ret = __split_huge_page_refcount(page, list);
+ BUG_ON(!ret && PageCompound(page));
+
+out_unlock:
+ anon_vma_unlock_write(anon_vma);
+ put_anon_vma(anon_vma);
+out:
+ if (ret)
+ count_vm_event(THP_SPLIT_PAGE_FAILED);
+ else
+ count_vm_event(THP_SPLIT_PAGE);
+ return ret;
}

#define VM_NO_THP (VM_SPECIAL | VM_HUGETLB | VM_SHARED | VM_MAYSHARE)
diff --git a/mm/swap.c b/mm/swap.c
index 2e647d4dc6bb..7b4fbb26cc2c 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -80,12 +80,86 @@ static void __put_compound_page(struct page *page)
(*dtor)(page);
}

+static inline bool compound_lock_needed(struct page *page)
+{
+ return IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
+ !PageSlab(page) && !PageHeadHuge(page);
+}
+
static void put_compound_page(struct page *page)
{
- struct page *page_head = compound_head(page);
+ struct page *page_head;
+ unsigned long flags;
+
+ if (likely(!PageTail(page))) {
+ if (put_page_testzero(page)) {
+ /*
+ * By the time all refcounts have been released
+ * split_huge_page cannot run anymore from under us.
+ */
+ if (PageHead(page))
+ __put_compound_page(page);
+ else
+ __put_single_page(page);
+ }
+ return;
+ }
+
+ /* __split_huge_page_refcount can run under us */
+ page_head = compound_head(page);
+
+ if (!compound_lock_needed(page_head)) {
+ /*
+ * If "page" is a THP tail, we must read the tail page flags
+ * after the head page flags. The split_huge_page side enforces
+ * write memory barriers between clearing PageTail and before
+ * the head page can be freed and reallocated.
+ */
+ smp_rmb();
+ if (likely(PageTail(page))) {
+ /* __split_huge_page_refcount cannot race here. */
+ VM_BUG_ON_PAGE(!PageHead(page_head), page_head);
+ VM_BUG_ON_PAGE(page_mapcount(page) != 0, page);
+ if (put_page_testzero(page_head)) {
+ /*
+ * If this is the tail of a slab compound page,
+ * the tail pin must not be the last reference
+ * held on the page, because the PG_slab cannot
+ * be cleared before all tail pins (which skips
+ * the _mapcount tail refcounting) have been
+ * released. For hugetlbfs the tail pin may be
+ * the last reference on the page instead,
+ * because PageHeadHuge will not go away until
+ * the compound page enters the buddy
+ * allocator.
+ */
+ VM_BUG_ON_PAGE(PageSlab(page_head), page_head);
+ __put_compound_page(page_head);
+ }
+ } else if (put_page_testzero(page))
+ __put_single_page(page);
+ return;
+ }

- if (put_page_testzero(page_head))
- __put_compound_page(page_head);
+ flags = compound_lock_irqsave(page_head);
+ /* here __split_huge_page_refcount won't run anymore */
+ if (likely(page != page_head && PageTail(page))) {
+ bool free;
+
+ free = put_page_testzero(page_head);
+ compound_unlock_irqrestore(page_head, flags);
+ if (free) {
+ if (PageHead(page_head))
+ __put_compound_page(page_head);
+ else
+ __put_single_page(page_head);
+ }
+ } else {
+ compound_unlock_irqrestore(page_head, flags);
+ VM_BUG_ON_PAGE(PageTail(page), page);
+ if (put_page_testzero(page))
+ __put_single_page(page);
+ }
}

void put_page(struct page *page)
@@ -97,6 +171,52 @@ void put_page(struct page *page)
}
EXPORT_SYMBOL(put_page);

+/*
+ * This function is exported but must not be called by anything other
+ * than get_page(). It implements the slow path of get_page().
+ */
+void __get_page_tail(struct page *page)
+{
+ struct page *page_head = compound_head(page);
+ unsigned long flags;
+
+ if (!compound_lock_needed(page_head)) {
+ smp_rmb();
+ if (likely(PageTail(page))) {
+ /*
+ * This is a hugetlbfs page or a slab page.
+ * __split_huge_page_refcount cannot race here.
+ */
+ VM_BUG_ON_PAGE(!PageHead(page_head), page_head);
+ VM_BUG_ON(page_head != page->first_page);
+ VM_BUG_ON_PAGE(atomic_read(&page_head->_count) <= 0,
+ page);
+ atomic_inc(&page_head->_count);
+ } else {
+ /*
+ * __split_huge_page_refcount run before us, "page" was
+ * a thp tail. the split page_head has been freed and
+ * reallocated as slab or hugetlbfs page of smaller
+ * order (only possible if reallocated as slab on x86).
+ */
+ VM_BUG_ON_PAGE(atomic_read(&page->_count) <= 0, page);
+ atomic_inc(&page->_count);
+ }
+ return;
+ }
+
+ flags = compound_lock_irqsave(page_head);
+ /* here __split_huge_page_refcount won't run anymore */
+ if (unlikely(page == page_head || !PageTail(page) ||
+ !get_page_unless_zero(page_head))) {
+ /* page is not part of THP page anymore */
+ VM_BUG_ON_PAGE(atomic_read(&page->_count) <= 0, page);
+ atomic_inc(&page->_count);
+ }
+ compound_unlock_irqrestore(page_head, flags);
+}
+EXPORT_SYMBOL(__get_page_tail);
+
/**
* put_pages_list() - release a list of pages
* @pages: list of pages threaded on page->lru
--
2.1.4


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