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Subject[PATCH 06 of 66] alter compound get_page/put_page
Date
From
From: Andrea Arcangeli <aarcange@redhat.com>

Alter compound get_page/put_page to keep references on subpages too, in order
to allow __split_huge_page_refcount to split an hugepage even while subpages
have been pinned by one of the get_user_pages() variants.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
---

diff --git a/arch/powerpc/mm/gup.c b/arch/powerpc/mm/gup.c
--- a/arch/powerpc/mm/gup.c
+++ b/arch/powerpc/mm/gup.c
@@ -16,6 +16,16 @@

#ifdef __HAVE_ARCH_PTE_SPECIAL

+static inline void pin_huge_page_tail(struct page *page)
+{
+ /*
+ * __split_huge_page_refcount() cannot run
+ * from under us.
+ */
+ VM_BUG_ON(atomic_read(&page->_count) < 0);
+ atomic_inc(&page->_count);
+}
+
/*
* The performance critical leaf functions are made noinline otherwise gcc
* inlines everything into a single function which results in too much
@@ -47,6 +57,8 @@ static noinline int gup_pte_range(pmd_t
put_page(page);
return 0;
}
+ if (PageTail(page))
+ pin_huge_page_tail(page);
pages[*nr] = page;
(*nr)++;

diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -105,6 +105,16 @@ static inline void get_head_page_multipl
atomic_add(nr, &page->_count);
}

+static inline void pin_huge_page_tail(struct page *page)
+{
+ /*
+ * __split_huge_page_refcount() cannot run
+ * from under us.
+ */
+ VM_BUG_ON(atomic_read(&page->_count) < 0);
+ atomic_inc(&page->_count);
+}
+
static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
unsigned long end, int write, struct page **pages, int *nr)
{
@@ -128,6 +138,8 @@ static noinline int gup_huge_pmd(pmd_t p
do {
VM_BUG_ON(compound_head(page) != head);
pages[*nr] = page;
+ if (PageTail(page))
+ pin_huge_page_tail(page);
(*nr)++;
page++;
refs++;
diff --git a/include/linux/mm.h b/include/linux/mm.h
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -351,9 +351,17 @@ static inline int page_count(struct page

static inline void get_page(struct page *page)
{
- page = compound_head(page);
- VM_BUG_ON(atomic_read(&page->_count) == 0);
+ VM_BUG_ON(atomic_read(&page->_count) < !PageTail(page));
atomic_inc(&page->_count);
+ if (unlikely(PageTail(page))) {
+ /*
+ * This is safe only because
+ * __split_huge_page_refcount can't run under
+ * get_page().
+ */
+ VM_BUG_ON(atomic_read(&page->first_page->_count) <= 0);
+ atomic_inc(&page->first_page->_count);
+ }
}

static inline struct page *virt_to_head_page(const void *x)
diff --git a/mm/swap.c b/mm/swap.c
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -56,17 +56,83 @@ static void __page_cache_release(struct
del_page_from_lru(zone, page);
spin_unlock_irqrestore(&zone->lru_lock, flags);
}
+}
+
+static void __put_single_page(struct page *page)
+{
+ __page_cache_release(page);
free_hot_cold_page(page, 0);
}

+static void __put_compound_page(struct page *page)
+{
+ compound_page_dtor *dtor;
+
+ __page_cache_release(page);
+ dtor = get_compound_page_dtor(page);
+ (*dtor)(page);
+}
+
static void put_compound_page(struct page *page)
{
- page = compound_head(page);
- if (put_page_testzero(page)) {
- compound_page_dtor *dtor;
-
- dtor = get_compound_page_dtor(page);
- (*dtor)(page);
+ if (unlikely(PageTail(page))) {
+ /* __split_huge_page_refcount can run under us */
+ struct page *page_head = page->first_page;
+ smp_rmb();
+ if (likely(PageTail(page) && get_page_unless_zero(page_head))) {
+ unsigned long flags;
+ if (unlikely(!PageHead(page_head))) {
+ /* PageHead is cleared after PageTail */
+ smp_rmb();
+ VM_BUG_ON(PageTail(page));
+ goto out_put_head;
+ }
+ /*
+ * Only run compound_lock on a valid PageHead,
+ * after having it pinned with
+ * get_page_unless_zero() above.
+ */
+ smp_mb();
+ /* page_head wasn't a dangling pointer */
+ compound_lock_irqsave(page_head, &flags);
+ if (unlikely(!PageTail(page))) {
+ /* __split_huge_page_refcount run before us */
+ compound_unlock_irqrestore(page_head, flags);
+ VM_BUG_ON(PageHead(page_head));
+ out_put_head:
+ if (put_page_testzero(page_head))
+ __put_single_page(page_head);
+ out_put_single:
+ if (put_page_testzero(page))
+ __put_single_page(page);
+ return;
+ }
+ VM_BUG_ON(page_head != page->first_page);
+ /*
+ * We can release the refcount taken by
+ * get_page_unless_zero now that
+ * split_huge_page_refcount is blocked on the
+ * compound_lock.
+ */
+ if (put_page_testzero(page_head))
+ VM_BUG_ON(1);
+ /* __split_huge_page_refcount will wait now */
+ VM_BUG_ON(atomic_read(&page->_count) <= 0);
+ atomic_dec(&page->_count);
+ VM_BUG_ON(atomic_read(&page_head->_count) <= 0);
+ compound_unlock_irqrestore(page_head, flags);
+ if (put_page_testzero(page_head))
+ __put_compound_page(page_head);
+ } else {
+ /* page_head is a dangling pointer */
+ VM_BUG_ON(PageTail(page));
+ goto out_put_single;
+ }
+ } else if (put_page_testzero(page)) {
+ if (PageHead(page))
+ __put_compound_page(page);
+ else
+ __put_single_page(page);
}
}

@@ -75,7 +141,7 @@ void put_page(struct page *page)
if (unlikely(PageCompound(page)))
put_compound_page(page);
else if (put_page_testzero(page))
- __page_cache_release(page);
+ __put_single_page(page);
}
EXPORT_SYMBOL(put_page);


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