mm: fix slab->page flags corruption

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 a race between compound_unlock() and slab
functions which perform page-flags updates.  This can occur when
get_page()/put_page() is called on a page from slab.

[akpm@linux-foundation.org: tweak comment text, fix comment layout, fix label indenting]
Reported-by: Amey Bhide <abhide@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Acked-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index aa20bafa40f627f646e75f481d84f0da9098b640..ce26716238c3632ba95d909bf69e1e4d4bc91da2 100644 (file)
--- 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 5c13f13389721fe60756ffb4dabe66d0c1e86e47..6fdd72ec15b01b4552480e02ad639de0a73d79de 100644 (file)
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -82,6 +82,25 @@ 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 so avoid taking
+                        * compound_lock().  Slab performs non-atomic bit ops
+                        * on page->flags for better performance.  In particular
+                        * slab_unlock() in slub used to be a hot path.  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
@@ -92,10 +111,10 @@ 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:
+out_put_single:
                                if (put_page_testzero(page))
                                        __put_single_page(page);
                                return;
@@ -115,6 +134,8 @@ 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 +183,18 @@ 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