linux-ext4 - Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking infrastructure and convert ext4 to use it

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Date:   Tue, 04 Apr 2017 12:25:46 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     NeilBrown <neilb@...e.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
        akpm@...ux-foundation.org, tytso@....edu, jack@...e.cz
Subject: Re: [RFC PATCH 0/4] fs: introduce new writeback error tracking
 infrastructure and convert ext4 to use it

On Tue, 2017-04-04 at 09:12 -0700, Matthew Wilcox wrote:
> On Tue, Apr 04, 2017 at 08:17:48AM -0400, Jeff Layton wrote:
> > Agreed that we should focus on POSIX compliance. I'll also note that
> > POSIX states:
> > 
> > "If more than one error occurs in processing a function call, any one
> > of the possible errors may be returned, as the order of
> > detection is undefined."
> > 
> >     http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_03
> > 
> > So, I'd like to push back on this idea that we need to prefer reporting
> > -EIO over other errors. POSIX certainly doesn't mandate that. 
> 
> I honestly wonder if we need to support ENOSPC from writeback at all.
> Looking at our history, the AS_EIO / AS_ENOSPC came from this patch
> in 2003:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/?id=fcad2b42fc2e15a94ba1a1ba8535681a735bfd16
> 
> That seems to come from here:
> http://lkml.iu.edu/hypermail/linux/kernel/0308.0/0205.html
> which is marked as a resend, but I can't find the original.
> 
> It's a little misleading because the immediately preceding patch
> introduced mapping->error, so there's no precedent here to speak of.
> It looks like we used to just silently lose writeback errors (*cough*).
> 
> I'd like to suggest that maybe we don't need to support multiple errors
> at all.  That all errors, including ENOSPC, get collapsed into EIO.
> POSIX already tells us to do that for close() and permits us to do that
> for fsync().
> 

That is certainly allowed under POSIX as I interpret the spec. At a
minimum we just need a single flag and can collapse all errors under
that.

That said, I think giving more specific errors where we can is useful.
When your program is erroring out and writing 'I/O error' to the logs,
then how much time will your admins burn before they figure out that it
really failed because the filesystem was full?
-- 
Jeff Layton <jlayton@...hat.com>