Re: [PATCH] fs: block pipe_write() on a frozen filesystem PING..

[Dmitry Monakhov <dmonakhov@openvz.org>]

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On Tue, 2 Apr 2013 20:57:27 +0100, Al Viro <viro@ZenIV.linux.org.uk> wrote:
> On Tue, Apr 02, 2013 at 04:38:22PM +0400, Dmitry Monakhov wrote:
> > On Thu, 27 Sep 2012 10:47:59 -0500, Eric Sandeen <sandeen@redhat.com> wrote:
> > > I noticed that this can sneak past a frozen filesystem:
> > Ping.. any body want to take care about that patch?
> 
> NAK.  Note that writing to FIFO that sits on filesystem that is outright
> read-only is allowed just fine; blocking it for frozen ones is bogus.
> 
> IOW, it should be treated the same way we treat touch_atime().  Moreover,
> it should only be done around file_update_time().  With trylock.
Ok fair enough.
Please take a look at updated version

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>From c01acb73b8dc9e7a891b8f087ad3eb53bb06d270 Mon Sep 17 00:00:00 2001
From: Dmitry Monakhov <dmonakhov@openvz.org>
Date: Wed, 3 Apr 2013 00:19:15 +0400
Subject: [PATCH] [PATCH] fs: skip mtime update for fifo on a frozen filesystem

Eric noticed that this can sneak past a frozen filesystem:
# mkfifo /mnt/test/fifo
# echo foo > /mnt/test/fifo &
# fsfreeze -f /mnt/test
# cat /mnt/test/fifo
and we get a warning that jbd2 has entered a transaction while frozen:
WARNING: at fs/ext4/super.c:240 ext4_journal_start_sb+0xce/0xe0 [ext4]() (Not tainted)
which is: WARN_ON(sb->s_writers.frozen == SB_FREEZE_COMPLETE);
and we get there via the file_update_time() path in pipe_write().
If filesystem is frozen we can block pipe_write() but this is not optimal
because fifo is special_file so let's skip mtime update as we do
with touch_atime()

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
---
 fs/pipe.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 2234f3f..62f63d5 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -654,10 +654,11 @@ out:
 		wake_up_interruptible_sync_poll(&pipe->wait, POLLIN | POLLRDNORM);
 		kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
 	}
-	if (ret > 0) {
+	if (ret > 0 && sb_start_write_trylock(inode->i_sb)) {
 		int err = file_update_time(filp);
 		if (err)
 			ret = err;
+		sb_end_write(inode->i_sb);
 	}
 	return ret;
 }
-- 
1.7.1