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From: Miklos Szeredi <>
To: Sargun Dhillon <>
Cc: overlayfs <>,
	Amir Goldstein <>,
	Alexander Viro <>,
	Giuseppe Scrivano <>,
	Vivek Goyal <>,
	Daniel J Walsh <>,,
	David Howells <>,
	Chengguang Xu <>,
	Christoph Hellwig <>, NeilBrown <>,
	Jan Kara <>, stable <>,
	Jeff Layton <>,
	Matthew Wilcox <>
Subject: Re: [PATCH v4] overlay: Implement volatile-specific fsync error behaviour
Date: Wed, 20 Jan 2021 16:08:55 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Fri, Jan 8, 2021 at 1:10 AM Sargun Dhillon <> wrote:
> Overlayfs's volatile option allows the user to bypass all forced sync calls
> to the upperdir filesystem. This comes at the cost of safety. We can never
> ensure that the user's data is intact, but we can make a best effort to
> expose whether or not the data is likely to be in a bad state.
> The best way to handle this in the time being is that if an overlayfs's
> upperdir experiences an error after a volatile mount occurs, that error
> will be returned on fsync, fdatasync, sync, and syncfs. This is
> contradictory to the traditional behaviour of VFS which fails the call
> once, and only raises an error if a subsequent fsync error has occurred,
> and been raised by the filesystem.
> One awkward aspect of the patch is that we have to manually set the
> superblock's errseq_t after the sync_fs callback as opposed to just
> returning an error from syncfs. This is because the call chain looks
> something like this:
> sys_syncfs ->
>         sync_filesystem ->
>                 __sync_filesystem ->
>                         /* The return value is ignored here
>                         sb->s_op->sync_fs(sb)
>                         _sync_blockdev
>                 /* Where the VFS fetches the error to raise to userspace */
>                 errseq_check_and_advance
> Because of this we call errseq_set every time the sync_fs callback occurs.
> Due to the nature of this seen / unseen dichotomy, if the upperdir is an
> inconsistent state at the initial mount time, overlayfs will refuse to
> mount, as overlayfs cannot get a snapshot of the upperdir's errseq that
> will increment on error until the user calls syncfs.

Thanks, this makes sense.  Queued for v4.11.


      parent reply	other threads:[~2021-01-20 15:13 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-08  0:10 [PATCH v4] overlay: Implement volatile-specific fsync error behaviour Sargun Dhillon
2021-01-08 15:11 ` Vivek Goyal
2021-01-19 11:49 ` Jeff Layton
2021-01-20  1:21   ` Sargun Dhillon
2021-01-20 15:08 ` Miklos Szeredi [this message]

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