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

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Date:   Thu, 12 Apr 2018 14:24:32 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Andres Freund <andres@...razel.de>,
        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 05:21:44PM -0400, Theodore Y. Ts'o wrote:
> On Thu, Apr 12, 2018 at 01:28:30PM -0700, Matthew Wilcox wrote:
> > 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.
> 
> When or how would the per-superblock wb_err flag get cleared?

That's not how errseq works, Ted ;-)

> Would all subsequent fsync() calls on that file system now return EIO?
> Or would only all subsequent syncfs() calls return EIO?

Only ones which occur after the last sampling get reported through this
particular file descriptor.