linux-ext4 - Re: [PATCH v7 16/22] block: convert to errseq

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Message-ID: <1498310166.4796.4.camel@redhat.com>
Date:   Sat, 24 Jun 2017 09:16:06 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...IV.linux.org.uk>, Jan Kara <jack@...e.cz>,
        tytso@....edu, axboe@...nel.dk, mawilcox@...rosoft.com,
        ross.zwisler@...ux.intel.com, corbet@....net,
        Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>,
        David Sterba <dsterba@...e.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Carlos Maiolino <cmaiolino@...hat.com>,
        Eryu Guan <eguan@...hat.com>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH v7 16/22] block: convert to errseq_t based writeback
 error tracking

On Sat, 2017-06-24 at 04:59 -0700, Christoph Hellwig wrote:
> On Tue, Jun 20, 2017 at 01:44:44PM -0400, Jeff Layton wrote:
> > In order to query for errors with errseq_t, you need a previously-
> > sampled point from which to check. When you call
> > filemap_write_and_wait_range though you don't have a struct file and so
> > no previously-sampled value.
> 
> So can we simply introduce variants of them that take a struct file?
> That would be:
> 
>  a) less churn
>  b) less code
>  c) less chance to get data integrity wrong

Yeah, I had that thought after I sent the reply to you earlier.

The main reason I didn't do that before was that I had myself convinced
that we needed to do the check_and_advance as late as possible in the
fsync process, after the metadata had been written.

Now that I think about it more, I think you're probably correct. As long
as we do the check and advance at some point after doing the
write_and_wait, we're fine here and shouldn't violate exactly once
semantics on the fsync return.
-- 
Jeff Layton <jlayton@...hat.com>