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

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Message-ID: <20180412213110.GF18364@bombadil.infradead.org>
Date:   Thu, 12 Apr 2018 14:31:10 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jeff Layton <jlayton@...hat.com>
Cc:     Andres Freund <andres@...razel.de>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Dave Chinner <david@...morbit.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 05:14:54PM -0400, Jeff Layton wrote:
> On Thu, 2018-04-12 at 13:28 -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.
> 
> Not a bad idea and shouldn't be too costly. mapping_set_error could
> flag the superblock one before or after the one in the mapping.
> 
> We'd need to define what happens if you interleave fsync and syncfs
> calls on the same inode though. How do we handle file->f_wb_err in that
> case? Would we need a second field in struct file to act as the per-sb
> error cursor?

Ooh.  I hadn't thought that through.  Bleh.  I don't want to add a field
to struct file for this uncommon case.

Maybe O_PATH could be used for this?  It gets you a file descriptor on
a particular filesystem, so syncfs() is defined, but it can't report
a writeback error.  So if you open something O_PATH, you can use the
file's f_wb_err for the mapping's error cursor.