This is a note to let you know that I've just added the patch titled

xfs: truncate pagecache before writeback in xfs_setattr_size()

to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: xfs-truncate-pagecache-before-writeback-in-xfs_setattr_size.patch and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

From foo@baz Mon Dec 18 14:12:35 CET 2017

From: Eryu Guan eguan@redhat.com Date: Wed, 1 Nov 2017 21:43:50 -0700 Subject: xfs: truncate pagecache before writeback in xfs_setattr_size()

From: Eryu Guan eguan@redhat.com

[ Upstream commit 350976ae21873b0d36584ea005076356431b8f79 ]

On truncate down, if new size is not block size aligned, we zero the rest of block to avoid exposing stale data to user, and iomap_truncate_page() skips zeroing if the range is already in unwritten state or a hole. Then we writeback from on-disk i_size to the new size if this range hasn't been written to disk yet, and truncate page cache beyond new EOF and set in-core i_size.

The problem is that we could write data between di_size and newsize before removing the page cache beyond newsize, as the extents may still be in unwritten state right after a buffer write. As such, the page of data that newsize lies in has not been zeroed by page cache invalidation before it is written, and xfs_do_writepage() hasn't triggered it's "zero data beyond EOF" case because we haven't updated in-core i_size yet. Then a subsequent mmap read could see non-zeros past EOF.

I occasionally see this in fsx runs in fstests generic/112, a simplified fsx operation sequence is like (assuming 4k block size xfs):

fallocate 0x0 0x1000 0x0 keep_size write 0x0 0x1000 0x0 truncate 0x0 0x800 0x1000 punch_hole 0x0 0x800 0x800 mapread 0x0 0x800 0x800

where fallocate allocates unwritten extent but doesn't update i_size, buffer write populates the page cache and extent is still unwritten, truncate skips zeroing page past new EOF and writes the page to disk, punch_hole invalidates the page cache, at last mapread reads the block back and sees non-zero beyond EOF.

Fix it by moving truncate_setsize() to before writeback so the page cache invalidation zeros the partial page at the new EOF. This also triggers "zero data beyond EOF" in xfs_do_writepage() at writeback time, because newsize has been set and page straddles the newsize.

Also fixed the wrong 'end' param of filemap_write_and_wait_range() call while we're at it, the 'end' is inclusive and should be 'newsize - 1'.

Suggested-by: Dave Chinner dchinner@redhat.com Signed-off-by: Eryu Guan eguan@redhat.com Acked-by: Dave Chinner dchinner@redhat.com Reviewed-by: Brian Foster bfoster@redhat.com Reviewed-by: Darrick J. Wong darrick.wong@oracle.com Signed-off-by: Darrick J. Wong darrick.wong@oracle.com Signed-off-by: Sasha Levin alexander.levin@verizon.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/xfs/xfs_iops.c | 36 ++++++++++++++++++++---------------- 1 file changed, 20 insertions(+), 16 deletions(-)

--- a/fs/xfs/xfs_iops.c +++ b/fs/xfs/xfs_iops.c @@ -871,22 +871,6 @@ xfs_setattr_size( return error;

/* - * We are going to log the inode size change in this transaction so - * any previous writes that are beyond the on disk EOF and the new - * EOF that have not been written out need to be written here. If we - * do not write the data out, we expose ourselves to the null files - * problem. Note that this includes any block zeroing we did above; - * otherwise those blocks may not be zeroed after a crash. - */ - if (did_zeroing || - (newsize > ip->i_d.di_size && oldsize != ip->i_d.di_size)) { - error = filemap_write_and_wait_range(VFS_I(ip)->i_mapping, - ip->i_d.di_size, newsize); - if (error) - return error; - } - - /* * We've already locked out new page faults, so now we can safely remove * pages from the page cache knowing they won't get refaulted until we * drop the XFS_MMAP_EXCL lock after the extent manipulations are @@ -902,9 +886,29 @@ xfs_setattr_size( * user visible changes). There's not much we can do about this, except * to hope that the caller sees ENOMEM and retries the truncate * operation. + * + * And we update in-core i_size and truncate page cache beyond newsize + * before writeback the [di_size, newsize] range, so we're guaranteed + * not to write stale data past the new EOF on truncate down. */ truncate_setsize(inode, newsize);

+ /* + * We are going to log the inode size change in this transaction so + * any previous writes that are beyond the on disk EOF and the new + * EOF that have not been written out need to be written here. If we + * do not write the data out, we expose ourselves to the null files + * problem. Note that this includes any block zeroing we did above; + * otherwise those blocks may not be zeroed after a crash. + */ + if (did_zeroing || + (newsize > ip->i_d.di_size && oldsize != ip->i_d.di_size)) { + error = filemap_write_and_wait_range(VFS_I(ip)->i_mapping, + ip->i_d.di_size, newsize - 1); + if (error) + return error; + } + error = xfs_trans_alloc(mp, &M_RES(mp)->tr_itruncate, 0, 0, 0, &tp); if (error) return error;

Patches currently in stable-queue which might be from eguan@redhat.com are

queue-4.9/ext4-fix-fdatasync-2-after-fallocate-2-operation.patch queue-4.9/xfs-truncate-pagecache-before-writeback-in-xfs_setattr_size.patch