mm: make page pfmemalloc check more robust

Commit c48a11c7ad26 ("netvm: propagate page->pfmemalloc to skb") added
checks for page->pfmemalloc to __skb_fill_page_desc():

        if (page->pfmemalloc && !page->mapping)
                skb->pfmemalloc = true;

It assumes page->mapping == NULL implies that page->pfmemalloc can be
trusted.  However, __delete_from_page_cache() can set set page->mapping
to NULL and leave page->index value alone.  Due to being in union, a
non-zero page->index will be interpreted as true page->pfmemalloc.

So the assumption is invalid if the networking code can see such a page.
And it seems it can.  We have encountered this with a NFS over loopback
setup when such a page is attached to a new skbuf.  There is no copying
going on in this case so the page confuses __skb_fill_page_desc which
interprets the index as pfmemalloc flag and the network stack drops
packets that have been allocated using the reserves unless they are to
be queued on sockets handling the swapping which is the case here and
that leads to hangs when the nfs client waits for a response from the
server which has been dropped and thus never arrive.

The struct page is already heavily packed so rather than finding another
hole to put it in, let's do a trick instead.  We can reuse the index
again but define it to an impossible value (-1UL).  This is the page
index so it should never see the value that large.  Replace all direct
users of page->pfmemalloc by page_is_pfmemalloc which will hide this
nastiness from unspoiled eyes.

The information will get lost if somebody wants to use page->index
obviously but that was the case before and the original code expected
that the information should be persisted somewhere else if that is
really needed (e.g.  what SLAB and SLUB do).

[akpm@linux-foundation.org: fix blooper in slub]
Fixes: c48a11c7ad26 ("netvm: propagate page->pfmemalloc to skb")
Signed-off-by: Michal Hocko <mhocko@suse.com>
Debugged-by: Vlastimil Babka <vbabka@suse.com>
Debugged-by: Jiri Bohac <jbohac@suse.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: David Miller <davem@davemloft.net>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: <stable@vger.kernel.org>	[3.6+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 33 insertions, 18 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2e872f92dbac..bf6f117fcf4d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1003,6 +1003,34 @@ static inline int page_mapped(struct page *page)
 }
 
 /*
+ * Return true only if the page has been allocated with
+ * ALLOC_NO_WATERMARKS and the low watermark was not
+ * met implying that the system is under some pressure.
+ */
+static inline bool page_is_pfmemalloc(struct page *page)
+{
+	/*
+	 * Page index cannot be this large so this must be
+	 * a pfmemalloc page.
+	 */
+	return page->index == -1UL;
+}
+
+/*
+ * Only to be called by the page allocator on a freshly allocated
+ * page.
+ */
+static inline void set_page_pfmemalloc(struct page *page)
+{
+	page->index = -1UL;
+}
+
+static inline void clear_page_pfmemalloc(struct page *page)
+{
+	page->index = 0;
+}
+
+/*
  * Different kinds of faults, as returned by handle_mm_fault().
  * Used to decide whether a process gets delivered SIGBUS or
  * just gets major/minor fault counters bumped up.
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 0038ac7466fd..15549578d559 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -63,15 +63,6 @@ struct page {
 		union {
 			pgoff_t index;		/* Our offset within mapping. */
 			void *freelist;		/* sl[aou]b first free object */
-			bool pfmemalloc;	/* If set by the page allocator,
-						 * ALLOC_NO_WATERMARKS was set
-						 * and the low watermark was not
-						 * met implying that the system
-						 * is under some pressure. The
-						 * caller should try ensure
-						 * this page is only used to
-						 * free other pages.
-						 */
 		};
 
 		union {
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 22b6d9ca1654..9b88536487e6 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1602,20 +1602,16 @@ static inline void __skb_fill_page_desc(struct sk_buff *skb, int i,
 	skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
 
 	/*
-	 * Propagate page->pfmemalloc to the skb if we can. The problem is
-	 * that not all callers have unique ownership of the page. If
-	 * pfmemalloc is set, we check the mapping as a mapping implies
-	 * page->index is set (index and pfmemalloc share space).
-	 * If it's a valid mapping, we cannot use page->pfmemalloc but we
-	 * do not lose pfmemalloc information as the pages would not be
-	 * allocated using __GFP_MEMALLOC.
+	 * Propagate page pfmemalloc to the skb if we can. The problem is
+	 * that not all callers have unique ownership of the page but rely
+	 * on page_is_pfmemalloc doing the right thing(tm).
 	 */
 	frag->page.p		  = page;
 	frag->page_offset	  = off;
 	skb_frag_size_set(frag, size);
 
 	page = compound_head(page);
-	if (page->pfmemalloc && !page->mapping)
+	if (page_is_pfmemalloc(page))
 		skb->pfmemalloc	= true;
 }
 
@@ -2263,7 +2259,7 @@ static inline struct page *dev_alloc_page(void)
 static inline void skb_propagate_pfmemalloc(struct page *page,
 					     struct sk_buff *skb)
 {
-	if (page && page->pfmemalloc)
+	if (page_is_pfmemalloc(page))
 		skb->pfmemalloc = true;
 }