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

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Date:   Mon, 26 Jun 2017 10:34:18 -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 09:16 -0400, Jeff Layton wrote:
> 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.

So I have a file_write_and_wait_range now that should DTRT for this
patch.

The bigger question is -- what about more complex filesystems like
ext4?  There are a couple of cases where we can return -EIO or -EROFS on
fsync before filemap_write_and_wait_range is ever called. Like this one
for instance:

        if (unlikely(ext4_forced_shutdown(EXT4_SB(inode->i_sb))))
                return -EIO;

...and the EXT4_MF_FS_ABORTED case.

Are those conditions ever recoverable, such that a later fsync could
succeed? IOW, could I do a remount or something such that the existing
fds are left open and become usable again? 

If so, then we really ought to advance the errseq_t in the file when we
catch those cases as well. If we have to do that, then it probably makes
sense to leave the ext4 patch as-is.
-- 
Jeff Layton <jlayton@...hat.com>