linux-ext4 - Re: fsync() errors is unsafe and risks data loss

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Date:   Thu, 12 Apr 2018 13:28:30 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Andres Freund <andres@...razel.de>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        Dave Chinner <david@...morbit.com>,
        Jeff Layton <jlayton@...hat.com>,
        Andreas Dilger <adilger@...ger.ca>,
        20180410184356.GD3563@...nk.org,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "Joshua D. Drake" <jd@...mandprompt.com>
Subject: Re: fsync() errors is unsafe and risks data loss

On Thu, Apr 12, 2018 at 01:13:22PM -0700, Andres Freund wrote:
> I think a per-file or even per-blockdev/fs error state that'd be
> returned by fsync() would be more than sufficient.

Ah; this was my suggestion to Jeff on IRC.  That we add a per-superblock
wb_err and then allow syncfs() to return it.  So you'd open an fd on
a directory (for example), and call syncfs() which would return -EIO
or -ENOSPC if either of those conditions had occurred since you opened
the fd.

>  I don't see that
> that'd realistically would trigger OOM or the inability to unmount a
> filesystem.

Ted's referring to the current state of affairs where the writeback error
is held in the inode; if we can't evict the inode because it's holding
the error indicator, that can send us OOM.  If instead we transfer the
error indicator to the superblock, then there's no problem.