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From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Date:   Mon, 7 Sep 2020 09:11:06 +0200
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Subject: Re: [PATCH] fsync.2: ERRORS: add EIO and ENOSPC
To:     milan.opensource@gmail.com
Cc:     lkml <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
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[Widening the CC to include Andrew and linux-fsdevel@]
[Milan: thanks for the patch, but it's unclear to me from your commit
message how/if you verified the details.]

Andrew, maybe you (or someone else) can comment, since long ago your

    commit f79e2abb9bd452d97295f34376dedbec9686b986
    Author: Andrew Morton <akpm@osdl.org>
    Date:   Fri Mar 31 02:30:42 2006 -0800

included a comment that is referred to in  stackoverflow discussion
about this topic (that SO discussion is in turn referred to by
https://bugzilla.kernel.org/show_bug.cgi?id=194757).

The essence as I understand it, is this:
(1) fsync() (and similar) may fail EIO or ENOSPC, at which point data
has not been synced.
(2) In this case, the EIO/ENOSPC setting is cleared so that...
(3) A subsequent fsync() might return success, but...
(4) That doesn't mean that the data in (1) landed on the disk.

The proposed manual page patch below wants to document this, but I'd
be happy to have an FS-knowledgeable person comment before I apply.

Thanks,

Michael

On Sat, 29 Aug 2020 at 09:13, <milan.opensource@gmail.com> wrote:
>
> From: Milan Shah <milan.opensource@gmail.com>
>
> This Fix addresses Bug 194757.
> Ref: https://bugzilla.kernel.org/show_bug.cgi?id=194757
> ---
>  man2/fsync.2 | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/man2/fsync.2 b/man2/fsync.2
> index 96401cd..f38b3e4 100644
> --- a/man2/fsync.2
> +++ b/man2/fsync.2
> @@ -186,6 +186,19 @@ In these cases disk caches need to be disabled using
>  or
>  .BR sdparm (8)
>  to guarantee safe operation.
> +
> +When
> +.BR fsync ()
> +or
> +.BR fdatasync ()
> +returns
> +.B EIO
> +or
> +.B ENOSPC
> +any error flags on pages in the file mapping are cleared, so subsequent synchronisation attempts
> +will return without error. It is
> +.I not
> +safe to retry synchronisation and assume that a non-error return means prior writes are now on disk.
>  .SH SEE ALSO
>  .BR sync (1),
>  .BR bdflush (2),
> --
> 2.7.4
>


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/