linux-ext4 - RE: [PATCH v8 12/18] Documentation: flesh out the section in vfs.txt on storing and reporting writeback errors

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Date:   Thu, 29 Jun 2017 18:21:50 +0000
From:   Matthew Wilcox <mawilcox@...rosoft.com>
To:     Jeff Layton <jlayton@...chiereds.net>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "jlayton@...nel.org" <jlayton@...nel.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...IV.linux.org.uk>, Jan Kara <jack@...e.cz>,
        "tytso@....edu" <tytso@....edu>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "ross.zwisler@...ux.intel.com" <ross.zwisler@...ux.intel.com>,
        "corbet@....net" <corbet@....net>, Chris Mason <clm@...com>,
        Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.com>,
        Carlos Maiolino <cmaiolino@...hat.com>,
        Eryu Guan <eguan@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Liu Bo <bo.li.liu@...cle.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: RE: [PATCH v8 12/18] Documentation: flesh out the section in vfs.txt
 on storing and reporting writeback errors

From: Jeff Layton [mailto:jlayton@...chiereds.net]
> On Thu, 2017-06-29 at 10:11 -0700, Darrick J. Wong wrote:
> > On Thu, Jun 29, 2017 at 09:19:48AM -0400, jlayton@...nel.org wrote:
> > > +Handling errors during writeback
> > > +--------------------------------
> > > +Most applications that utilize the pagecache will periodically call
> > > +fsync to ensure that data written has made it to the backing store.
> >
> > /me wonders if this sentence ought to be worded more strongly, e.g.
> >
> > "Applications that utilize the pagecache must call a data
> > synchronization syscall such as fsync, fdatasync, or msync to ensure
> > that data written has made it to the backing store."
> 
> Well...only if they care about the data. There are some that don't. :)

Also, applications don't "utilize the pagecache"; filesystems use the pagecache.
Applications may or may not use cached I/O.  How about this:

Applications which care about data integrity and use cached I/O will
periodically call fsync(), msync() or fdatasync() to ensure that their
data is durable.

> What should we do about sync_file_range here? It doesn't currently call
> any filesystem operations directly, so we don't have a good way to make
> it selectively use errseq_t handling there.
> 
> I could resurrect the FS_* flag for that, though I don't really like
> that. Should I just go ahead and convert it over to use errseq_t under
> the theory that most callers will eventually want that anyway?

I think so.