From 9fe55eea7e4b444bafc42fa0000cc2d1d2847275 Mon Sep 17 00:00:00 2001 From: Steven Whitehouse Date: Fri, 24 Jan 2014 14:42:22 +0000 Subject: [PATCH] Fix race when checking i_size on direct i/o read So far I've had one ACK for this, and no other comments. So I think it is probably time to send this via some suitable tree. I'm guessing that the vfs tree would be the most appropriate route, but not sure that there is one at the moment (don't see anything recent at kernel.org) so in that case I think -mm is the "back up plan". Al, please let me know if you will take this? Steve. --------------------- Following on from the "Re: [PATCH v3] vfs: fix a bug when we do some dio reads with append dio writes" thread on linux-fsdevel, this patch is my current version of the fix proposed as option (b) in that thread. Removing the i_size test from the direct i/o read path at vfs level means that filesystems now have to deal with requests which are beyond i_size themselves. These I've divided into three sets: a) Those with "no op" ->direct_IO (9p, cifs, ceph) These are obviously not going to be an issue b) Those with "home brew" ->direct_IO (nfs, fuse) I've been told that NFS should not have any problem with the larger i_size, however I've added an extra test to FUSE to duplicate the original behaviour just to be on the safe side. c) Those using __blockdev_direct_IO() These call through to ->get_block() which should deal with the EOF condition correctly. I've verified that with GFS2 and I believe that Zheng has verified it for ext4. I've also run the test on XFS and it passes both before and after this change. The part of the patch in filemap.c looks a lot larger than it really is - there are only two lines of real change. The rest is just indentation of the contained code. There remains a test of i_size though, which was added for btrfs. It doesn't cause the other filesystems a problem as the test is performed after ->direct_IO has been called. It is possible that there is a race that does matter to btrfs, however this patch doesn't change that, so its still an overall improvement. Signed-off-by: Steven Whitehouse Reported-by: Zheng Liu Cc: Jan Kara Cc: Dave Chinner Acked-by: Miklos Szeredi Cc: Chris Mason Cc: Josef Bacik Cc: Christoph Hellwig Cc: Alexander Viro Signed-off-by: Al Viro --- fs/fuse/file.c | 3 +++ mm/filemap.c | 42 ++++++++++++++++++++---------------------- 2 files changed, 23 insertions(+), 22 deletions(-) diff --git a/fs/fuse/file.c b/fs/fuse/file.c index 7e70506297bc..89fdfd1919af 100644 --- a/fs/fuse/file.c +++ b/fs/fuse/file.c @@ -2710,6 +2710,9 @@ fuse_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov, inode = file->f_mapping->host; i_size = i_size_read(inode); + if ((rw == READ) && (offset > i_size)) + return 0; + /* optimization for short read */ if (async_dio && rw != WRITE && offset + count > i_size) { if (offset >= i_size) diff --git a/mm/filemap.c b/mm/filemap.c index b7749a92021c..01842867c9d2 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1428,30 +1428,28 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov, if (!count) goto out; /* skip atime */ size = i_size_read(inode); - if (pos < size) { - retval = filemap_write_and_wait_range(mapping, pos, + retval = filemap_write_and_wait_range(mapping, pos, pos + iov_length(iov, nr_segs) - 1); - if (!retval) { - retval = mapping->a_ops->direct_IO(READ, iocb, - iov, pos, nr_segs); - } - if (retval > 0) { - *ppos = pos + retval; - count -= retval; - } + if (!retval) { + retval = mapping->a_ops->direct_IO(READ, iocb, + iov, pos, nr_segs); + } + if (retval > 0) { + *ppos = pos + retval; + count -= retval; + } - /* - * Btrfs can have a short DIO read if we encounter - * compressed extents, so if there was an error, or if - * we've already read everything we wanted to, or if - * there was a short read because we hit EOF, go ahead - * and return. Otherwise fallthrough to buffered io for - * the rest of the read. - */ - if (retval < 0 || !count || *ppos >= size) { - file_accessed(filp); - goto out; - } + /* + * Btrfs can have a short DIO read if we encounter + * compressed extents, so if there was an error, or if + * we've already read everything we wanted to, or if + * there was a short read because we hit EOF, go ahead + * and return. Otherwise fallthrough to buffered io for + * the rest of the read. + */ + if (retval < 0 || !count || *ppos >= size) { + file_accessed(filp); + goto out; } }