linux-ext4 - Re: [RFC PATCH 1/4] fs: new infrastructure for writeback error handling and reporting

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Date:   Mon, 3 Apr 2017 09:15:47 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jeff Layton <jlayton@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ext4@...r.kernel.org, akpm@...ux-foundation.org,
        tytso@....edu, jack@...e.cz, neilb@...e.com
Subject: Re: [RFC PATCH 1/4] fs: new infrastructure for writeback error
 handling and reporting

On Mon, Apr 03, 2017 at 11:19:51AM -0400, Jeff Layton wrote:
> Yes, so just to be clear here if you bump a 32 bit counter every
> microsecond you'll end up wrapping in a little over an hour. How fast
> can DAX generate I/O errors? :)

I admit to not having picked through the code, but how often do we try
to do writebacks?  And how often do we retry writebacks once an -EIO
has happened?  Once we mark a page as PG_error, do we keep trying to
write it back and set the AS error each time?

> I'm fine with a 32 bit counter (and even with using the low order bits
> to store error flags) if we're ok with that limitation. The big
> question there is whether it's ok to continue reporting -EIO when there
> has actually been nothing but -ENOSPC errors since the last fsync. I
> think it's a corner case that's not of terribly great concern so I'm
> fine with that.

Yeah, I was thinking about that, and I'm fine with it too.

> We could try to mitigate it by zeroing out the value when i_writecount
> goes to zero though. Then if you close all of the fds on the file, the
> error is cleared. Or maybe we could add a new ioctl to explicitly zero
> it out?

I'm OK with zeroing the wb_err once i_writecount drops to 0.  Everybody
who cares has already been notified.  The new ioctl feels like overkill.