[patch 3/3] NFS: Fix a writeback race...

[Greg Kroah-Hartman <gregkh@suse.de>]

[-- Attachment #1: nfs-fix-a-writeback-race.patch --]
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-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Trond Myklebust <Trond.Myklebust@netapp.com>

patch 61e930a904966cc37e0a3404276f0b73037e57ca in mainline

This patch fixes a regression that was introduced by commit
44dd151d5c21234cc534c47d7382f5c28c3143cd

We cannot zero the user page in nfs_mark_uptodate() any more, since

  a) We'd be modifying the page without holding the page lock
  b) We can race with other updates of the page, most notably
     because of the call to nfs_wb_page() in nfs_writepage_setup().

Instead, we do the zeroing in nfs_update_request() if we see that we're
creating a request that might potentially be marked as up to date.

Thanks to Olivier Paquet for reporting the bug and providing a test-case.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
 fs/nfs/write.c |   17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -167,8 +167,6 @@ static void nfs_mark_uptodate(struct pag
 		return;
 	if (count != nfs_page_length(page))
 		return;
-	if (count != PAGE_CACHE_SIZE)
-		zero_user_page(page, count, PAGE_CACHE_SIZE - count, KM_USER0);
 	SetPageUptodate(page);
 }
 
@@ -643,7 +641,8 @@ static struct nfs_page * nfs_update_requ
 				return ERR_PTR(error);
 			}
 			spin_unlock(&inode->i_lock);
-			return new;
+			req = new;
+			goto zero_page;
 		}
 		spin_unlock(&inode->i_lock);
 
@@ -671,13 +670,23 @@ static struct nfs_page * nfs_update_requ
 	if (offset < req->wb_offset) {
 		req->wb_offset = offset;
 		req->wb_pgbase = offset;
-		req->wb_bytes = rqend - req->wb_offset;
+		req->wb_bytes = max(end, rqend) - req->wb_offset;
+		goto zero_page;
 	}
 
 	if (end > rqend)
 		req->wb_bytes = end - req->wb_offset;
 
 	return req;
+zero_page:
+	/* If this page might potentially be marked as up to date,
+	 * then we need to zero any uninitalised data. */
+	if (req->wb_pgbase == 0 && req->wb_bytes != PAGE_CACHE_SIZE
+			&& !PageUptodate(req->wb_page))
+		zero_user_page(req->wb_page, req->wb_bytes,
+				PAGE_CACHE_SIZE - req->wb_bytes,
+				KM_USER0);
+	return req;
 }
 
 int nfs_flush_incompatible(struct file *file, struct page *page)

--