From: Giuseppe Scrivano <gscrivan@redhat.com>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Vivek Goyal <vgoyal@redhat.com>,
Sargun Dhillon <sargun@sargun.me>,
overlayfs <linux-unionfs@vger.kernel.org>,
Miklos Szeredi <miklos@szeredi.hu>,
Daniel J Walsh <dwalsh@redhat.com>
Subject: Re: [PATCH v7] overlayfs: Provide a mount option "volatile" to skip sync
Date: Mon, 09 Nov 2020 09:53:01 +0100 [thread overview]
Message-ID: <87k0uulxn6.fsf@redhat.com> (raw)
In-Reply-To: <CAOQ4uxhyzw=fHokRuCDFwD7SUg14_i1W0HMp9AGD6UxC5t5+tQ@mail.gmail.com> (Amir Goldstein's message of "Sat, 7 Nov 2020 11:35:04 +0200")
> On Fri, Nov 6, 2020 at 9:43 PM Giuseppe Scrivano <gscrivan@redhat.com> wrote:
>>
>> 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
>>
>
> FWIW, the sync -f command above is
> 1. Not needed when re-mounting overlayfs as volatile
> 2. Not enough when re-mounting overlayfs as non-volatile
>
> In the latter case, a full sync (no -f) is required.
Thanks for the clarification. Why wouldn't a syncfs on the upper
directory be enough to ensure files are persisted and safe to reuse
after a crash?
Regards,
Giuseppe
next prev parent reply other threads:[~2020-11-09 8:53 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
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 [this message]
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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