From: Giuseppe Scrivano <gscrivan@redhat.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: Sargun Dhillon <sargun@sargun.me>,
overlayfs <linux-unionfs@vger.kernel.org>,
Miklos Szeredi <miklos@szeredi.hu>,
Amir Goldstein <amir73il@gmail.com>,
Daniel J Walsh <dwalsh@redhat.com>
Subject: Re: [PATCH v7] overlayfs: Provide a mount option "volatile" to skip sync
Date: Fri, 06 Nov 2020 20:42:55 +0100 [thread overview]
Message-ID: <87o8kamfuo.fsf@redhat.com> (raw)
In-Reply-To: <20201106190325.GB1445528@redhat.com> (Vivek Goyal's message of "Fri, 6 Nov 2020 14:03:25 -0500")
Vivek Goyal <vgoyal@redhat.com> writes:
> On Fri, Nov 06, 2020 at 09:58:39AM -0800, Sargun Dhillon wrote:
>
> [..]
>> There is some slightly confusing behaviour here [I realize this
>> behaviour is as intended]:
>>
>> (root) ~ # mount -t overlay -o
>> volatile,index=off,lowerdir=/root/lowerdir,upperdir=/root/upperdir,workdir=/root/workdir
>> none /mnt/foo
>> (root) ~ # umount /mnt/foo
>> (root) ~ # mount -t overlay -o
>> volatile,index=off,lowerdir=/root/lowerdir,upperdir=/root/upperdir,workdir=/root/workdir
>> none /mnt/foo
>> mount: /mnt/foo: wrong fs type, bad option, bad superblock on none,
>> missing codepage or helper program, or other error.
>>
>> From my understanding, the dirty flag should only be a problem if the
>> existing overlayfs is unmounted uncleanly. Docker does
>> this (mount, and re-mounts) during startup time because it writes some
>> files to the overlayfs. I think that we should harden
>> the volatile check slightly, and make it so that within the same boot,
>> it's not a problem, and having to have the user clear
>> the workdir every time is a pain. In addition, the semantics of the
>> volatile patch itself do not appear to be such that they
>> would break mounts during the same boot / mount of upperdir -- as
>> overlayfs does not defer any writes in itself, and it's
>> only that it's short-circuiting writes to the upperdir.
>
> umount does a sync normally and with "volatile" overlayfs skips that
> sync. So a successful unmount does not mean that file got synced
> to backing store. It is possible, after umount, system crashed
> and after reboot, user tried to mount upper which is corrupted
> now and overlay will not detect it.
>
> You seem to be asking for an alternate option where we disable
> fsync() but not syncfs. In that case sync on umount will still
> be done. And that means a successful umount should mean upper
> is fine and it could automatically remove incomapt dir upon
> umount.
could this be handled in user space? It should still be possible to do
the equivalent of:
# sync -f /root/upperdir
# rm -rf /root/workdir/incompat/volatile
Regards,
Giuseppe
next prev parent reply other threads:[~2020-11-06 19:43 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-31 18:15 [PATCH v7] overlayfs: Provide a mount option "volatile" to skip sync Vivek Goyal
2020-09-01 8:22 ` Amir Goldstein
2020-09-01 13:14 ` Vivek Goyal
2020-11-06 17:58 ` Sargun Dhillon
2020-11-06 19:00 ` Amir Goldstein
2020-11-06 19:20 ` Vivek Goyal
2020-11-09 17:22 ` Vivek Goyal
2020-11-09 17:25 ` Sargun Dhillon
2020-11-09 19:39 ` Amir Goldstein
2020-11-09 20:24 ` Vivek Goyal
2020-11-06 19:03 ` Vivek Goyal
2020-11-06 19:42 ` Giuseppe Scrivano [this message]
2020-11-07 9:35 ` Amir Goldstein
2020-11-07 11:52 ` Sargun Dhillon
2020-11-09 20:40 ` Vivek Goyal
2020-11-09 8:53 ` Giuseppe Scrivano
2020-11-09 10:10 ` Amir Goldstein
2020-11-09 16:36 ` Vivek Goyal
2020-11-09 17:09 ` Vivek Goyal
2020-11-09 17:20 ` Amir Goldstein
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