[ 3.5.y.z extended stable ] Patch "nfs: don't extend writes to cover entire page if pagecache" has been added to staging queue

[Herton Ronaldo Krzesinski]

This is a note to let you know that I have just added a patch titled

    nfs: don't extend writes to cover entire page if pagecache

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Herton

------

>From 7639b0ccda91a45b0a378e2fc69c1aecd936b35f Mon Sep 17 00:00:00 2001
From: Jeff Layton <jlayton at redhat.com>
Date: Mon, 10 Dec 2012 09:25:48 -0500
Subject: [PATCH] nfs: don't extend writes to cover entire page if pagecache
 is invalid

commit 81d9bce5309288086b58b4d97a644e495fef75f2 upstream.

Jian reported that the following sequence would leave "testfile" with
corrupt data:

    # mount localhost:/export /mnt/nfs/ -o vers=3
    # echo abc > /mnt/nfs/testfile; echo def >> /export/testfile; echo ghi >> /mnt/nfs/testfile
    # cat -v /export/testfile
    abc
    ^@^@^@^@ghi

While there's no locking involved here, the operations are serialized,
so CTO should prevent corruption.

The first write to the file is fine and writes 4 bytes. The file is then
extended on the server. When it's reopened a GETATTR is issued and the
size change is noticed. This causes NFS_INO_INVALID_DATA to be set on
the file. Because the file is opened for write only,
nfs_want_read_modify_write() returns 0 to nfs_write_begin().
nfs_updatepage then calls nfs_write_pageuptodate() to see if it should
extend the nfs_page to cover the whole page. NFS_INO_INVALID_DATA is
still set on the file at that point, but that flag is ignored and
nfs_pageuptodate erroneously extends the write to cover the whole page,
with the write done on the server side filled in with zeroes.

This patch just has that function check for NFS_INO_INVALID_DATA in
addition to NFS_INO_REVAL_PAGECACHE. This fixes the bug, but looking
over the code, I wonder if we might have a similar bug in
nfs_revalidate_size(). The difference between those two flags is very
subtle, so it seems like we ought to be checking for
NFS_INO_INVALID_DATA in most of the places that we look for
NFS_INO_REVAL_PAGECACHE.

I believe this is regression introduced by commit 8d197a568. The code
did check for NFS_INO_INVALID_DATA prior to that patch.

Original bug report is here:

    https://bugzilla.redhat.com/show_bug.cgi?id=885743

Reported-by: Jian Li <jiali at redhat.com>
Signed-off-by: Jeff Layton <jlayton at redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust at netapp.com>
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski at canonical.com>
---
 fs/nfs/write.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index c11d7cf..bce88ac 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -855,7 +855,7 @@ static bool nfs_write_pageuptodate(struct page *page, struct inode *inode)
 {
 	if (nfs_have_delegated_attributes(inode))
 		goto out;
-	if (NFS_I(inode)->cache_validity & NFS_INO_REVAL_PAGECACHE)
+	if (NFS_I(inode)->cache_validity & (NFS_INO_INVALID_DATA|NFS_INO_REVAL_PAGECACHE))
 		return false;
 out:
 	return PageUptodate(page) != 0;
--
1.7.9.5