[ 092/171] ceph: check PG_Private flag before accessing page->private

[Greg Kroah-Hartman]

3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Yan, Zheng" <zheng.z.yan@xxxxxxxxx>

(cherry picked from commit 28c0254ede13ab575d2df5c6585ed3d4817c3e6b)

I got lots of NULL pointer dereference Oops when compiling kernel on ceph.
The bug is because the kernel page migration routine replaces some pages
in the page cache with new pages, these new pages' private can be non-zero.

Signed-off-by: Zheng Yan <zheng.z.yan@xxxxxxxxx>
Signed-off-by: Sage Weil <sage@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/ceph/addr.c |   21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

--- a/fs/ceph/addr.c
+++ b/fs/ceph/addr.c
@@ -54,7 +54,12 @@
 	(CONGESTION_ON_THRESH(congestion_kb) -				\
 	 (CONGESTION_ON_THRESH(congestion_kb) >> 2))
 
-
+static inline struct ceph_snap_context *page_snap_context(struct page *page)
+{
+	if (PagePrivate(page))
+		return (void *)page->private;
+	return NULL;
+}
 
 /*
  * Dirty a page.  Optimistically adjust accounting, on the assumption
@@ -142,10 +147,9 @@ static void ceph_invalidatepage(struct p
 {
 	struct inode *inode;
 	struct ceph_inode_info *ci;
-	struct ceph_snap_context *snapc = (void *)page->private;
+	struct ceph_snap_context *snapc = page_snap_context(page);
 
 	BUG_ON(!PageLocked(page));
-	BUG_ON(!page->private);
 	BUG_ON(!PagePrivate(page));
 	BUG_ON(!page->mapping);
 
@@ -182,7 +186,6 @@ static int ceph_releasepage(struct page
 	struct inode *inode = page->mapping ? page->mapping->host : NULL;
 	dout("%p releasepage %p idx %lu\n", inode, page, page->index);
 	WARN_ON(PageDirty(page));
-	WARN_ON(page->private);
 	WARN_ON(PagePrivate(page));
 	return 0;
 }
@@ -443,7 +446,7 @@ static int writepage_nounlock(struct pag
 	osdc = &fsc->client->osdc;
 
 	/* verify this is a writeable snap context */
-	snapc = (void *)page->private;
+	snapc = page_snap_context(page);
 	if (snapc == NULL) {
 		dout("writepage %p page %p not dirty?\n", inode, page);
 		goto out;
@@ -451,7 +454,7 @@ static int writepage_nounlock(struct pag
 	oldest = get_oldest_context(inode, &snap_size);
 	if (snapc->seq > oldest->seq) {
 		dout("writepage %p page %p snapc %p not writeable - noop\n",
-		     inode, page, (void *)page->private);
+		     inode, page, snapc);
 		/* we should only noop if called by kswapd */
 		WARN_ON((current->flags & PF_MEMALLOC) == 0);
 		ceph_put_snap_context(oldest);
@@ -591,7 +594,7 @@ static void writepages_finish(struct cep
 			clear_bdi_congested(&fsc->backing_dev_info,
 					    BLK_RW_ASYNC);
 
-		ceph_put_snap_context((void *)page->private);
+		ceph_put_snap_context(page_snap_context(page));
 		page->private = 0;
 		ClearPagePrivate(page);
 		dout("unlocking %d %p\n", i, page);
@@ -795,7 +798,7 @@ get_more_pages:
 			}
 
 			/* only if matching snap context */
-			pgsnapc = (void *)page->private;
+			pgsnapc = page_snap_context(page);
 			if (pgsnapc->seq > snapc->seq) {
 				dout("page snapc %p %lld > oldest %p %lld\n",
 				     pgsnapc, pgsnapc->seq, snapc, snapc->seq);
@@ -984,7 +987,7 @@ retry_locked:
 	BUG_ON(!ci->i_snap_realm);
 	down_read(&mdsc->snap_rwsem);
 	BUG_ON(!ci->i_snap_realm->cached_context);
-	snapc = (void *)page->private;
+	snapc = page_snap_context(page);
 	if (snapc && snapc != ci->i_head_snapc) {
 		/*
 		 * this page is already dirty in another (older) snap


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