From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: Ira Weiny <ira.weiny@intel.com>
Cc: Ext4 Developers List <linux-ext4@vger.kernel.org>,
syzkaller-bugs@googlegroups.com, stable@kernel.org,
syzbot+bca9799bf129256190da@syzkaller.appspotmail.com
Subject: Re: [PATCH] ext4: reject mount options not supported when remounting in handle_mount_opt()
Date: Wed, 15 Apr 2020 18:07:52 -0400 [thread overview]
Message-ID: <20200415220752.GA5187@mit.edu> (raw)
In-Reply-To: <20200415202537.GA2309605@iweiny-DESK2.sc.intel.com>
On Wed, Apr 15, 2020 at 01:25:37PM -0700, Ira Weiny wrote:
> This fundamentally changes the behavior from forcing the dax mode to be the
> same across the remount to only failing if we are going from non-dax to dax,
> adding -o dax on the remount?
>
> But going from -o dax to 'not -o dax' would be ok?
>
> FWIW after thinking about it some I _think_ it would be ok to allow the dax
> mode to change on a remount and let the inodes in memory stay in the mode they
> are at. And newly loaded inodes would get the new mode... Unfortunately
> without the STATX patch I have proposed the user does not have any way of
> knowing which files are in which mode.
We don't currently support mount -o nodax. So the intention of the
current code is that the dax mode can't change in either direction
(enabling or disabling) as a remount option.
The syzkaller report was because changing dax mode racing with other
operations given the current code base, could result in a kernel OOPS.
So we *do* need to rule it out at least for now.
I certainly don't object to allowing changing dax mode as a remount
--- so long as we have tests to make sure that if we stress opening,
reading, writing, mmap'ing files, etc., while another thread is
flipping back and forth between dax=never and dax=always is mount -o
remount --- and make sure that we don't end up crashing.
And this test needs to be in xfstests, because trying to figure out
what triggers a syzkaller failures in file system land is a pain in
the *ss so we really want a dedicated xfstests for this case. Have
you tested your patch series to make sure we don't have some potential
races here?
- Ted
next prev parent reply other threads:[~2020-04-15 22:08 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <to=00000000000098a5d505a34d1e48@google.com>
2020-04-15 17:48 ` [PATCH] ext4: reject mount options not supported when remounting in handle_mount_opt() Theodore Ts'o
2020-04-15 18:12 ` Andreas Dilger
2020-04-15 20:25 ` Ira Weiny
2020-04-15 22:07 ` Theodore Y. Ts'o [this message]
2020-04-16 5:23 ` Ira Weiny
2020-04-22 16:10 ` Jan Kara
2020-05-14 14:34 ` Theodore Y. Ts'o
2020-05-16 1:49 ` Ira Weiny
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