[PATCH v4] mm: Fix slab->page flags corruption.

[Pravin B Shelar]

v3-v4:
	- Added comments.
	- Removed VM_BUG_ON on PageHead from put_page.
v2-v3:
	- Check if page is still compound page after inc refcnt.
v1-v2:
	- Avoid taking compound lock for slab pages.

--8<--------------------------cut here-------------------------->8--

Transparent huge pages can change page->flags (PG_compound_lock)
without taking Slab lock. Since THP can not break slab pages we can
safely access compound page without taking compound lock.

Specifically this patch fixes race between compound_unlock and slab
functions which does page-flags update. This can occur when
get_page/put_page is called on page from slab object.

Reported-by: Amey Bhide <abhide@xxxxxxxxxx>
Signed-off-by: Pravin B Shelar <pshelar@xxxxxxxxxx>
Reviewed-by: Christoph Lameter <cl@xxxxxxxxx>
---
 include/linux/mm.h |    2 ++
 mm/swap.c          |   34 +++++++++++++++++++++++++++++++++-
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8437e93..ddd58ce 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -321,6 +321,7 @@ static inline int is_vmalloc_or_module_addr(const void *x)
 static inline void compound_lock(struct page *page)
 {
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	VM_BUG_ON(PageSlab(page));
 	bit_spin_lock(PG_compound_lock, &page->flags);
 #endif
 }
@@ -328,6 +329,7 @@ static inline void compound_lock(struct page *page)
 static inline void compound_unlock(struct page *page)
 {
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	VM_BUG_ON(PageSlab(page));
 	bit_spin_unlock(PG_compound_lock, &page->flags);
 #endif
 }
diff --git a/mm/swap.c b/mm/swap.c
index 8ff73d8..93c709b 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -82,6 +82,24 @@ static void put_compound_page(struct page *page)
 		if (likely(page != page_head &&
 			   get_page_unless_zero(page_head))) {
 			unsigned long flags;
+
+			/* THP can not break up slab pages, avoid taking
+			 * compound_lock(). Slab prefer non atomic bit ops
+			 * on page->flags for better performance. In particular
+			 * slab_unlock() in slub used to be a hot path
+			 * item. It is still hot on arches that do not support
+			 * this_cpu_cmpxchg_double. */
+
+			if (PageSlab(page_head)) {
+				if (PageTail(page)) {
+					if (put_page_testzero(page_head))
+						VM_BUG_ON(1);
+
+					atomic_dec(&page->_mapcount);
+					goto skip_lock_tail;
+				} else
+					goto skip_lock;
+			}
 			/*
 			 * page_head wasn't a dangling pointer but it
 			 * may not be a head page anymore by the time
@@ -92,7 +110,7 @@ static void put_compound_page(struct page *page)
 			if (unlikely(!PageTail(page))) {
 				/* __split_huge_page_refcount run before us */
 				compound_unlock_irqrestore(page_head, flags);
-				VM_BUG_ON(PageHead(page_head));
+			skip_lock:
 				if (put_page_testzero(page_head))
 					__put_single_page(page_head);
 			out_put_single:
@@ -115,6 +133,8 @@ static void put_compound_page(struct page *page)
 			VM_BUG_ON(atomic_read(&page_head->_count) <= 0);
 			VM_BUG_ON(atomic_read(&page->_count) != 0);
 			compound_unlock_irqrestore(page_head, flags);
+
+			skip_lock_tail:
 			if (put_page_testzero(page_head)) {
 				if (PageHead(page_head))
 					__put_compound_page(page_head);
@@ -162,6 +182,18 @@ bool __get_page_tail(struct page *page)
 	struct page *page_head = compound_trans_head(page);
 
 	if (likely(page != page_head && get_page_unless_zero(page_head))) {
+
+		/* Ref to put_compound_page() comment. */
+		if (PageSlab(page_head)) {
+			if (likely(PageTail(page))) {
+				__get_page_tail_foll(page, false);
+				return true;
+			} else {
+				put_page(page_head);
+				return false;
+			}
+		}
+
 		/*
 		 * page_head wasn't a dangling pointer but it
 		 * may not be a head page anymore by the time
-- 
1.7.10

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