Re: [PATCH 0/2 V4] Resubmit items failed during writeback

[Brian Foster <bfoster@redhat.com>]

On Fri, Jun 30, 2017 at 01:09:13PM +0200, Carlos Maiolino wrote:
> Hey,
> 
> > Taking a quick look at something I have laying around that you sent
> > previously (I assume the test hasn't changed much), I see we basically
> > create an overprovisioned dm-thin vol, mount it and dio write to it
> > until we start to see async write failures. So is the purpose of the
> > wait that we need the AIL to push and ultimately fail the associated
> > buffers before we attempt an unmount? If so, I'm wondering if you could
> > xfs_freeze the fs rather than wait (it looks like freeze sync pushes the
> > AIL)..?
> > 
> 
> As we discussed off-list, yes, I'd done some testing, and I believe we can use
> freezing to test it.
> 
> > > > > 
> > > > > I think we all agree that generic error injection (which Darrick has
> > > > > started playing with, but I haven't looked at yet) doesn't need to be
> > > > > bundled with this series (I hope we didn't scare you there ;). What I
> > > > > was asking for is a single patch that adds error injection in one spot
> > > > > with a configurable frequency. I'll refer to commit 609adfc2ed ("xfs:
> > > > > debug mode log record crc error injection") again because it is a simple
> > > > > example of a small DEBUG only hunk of code and boilerplate code to add a
> > > > > sysfs knob.
> > > > > 
> > > 
> > > I'll keep this in mind, and try to work on something like that :)
> > > 
> > 
> > I think error injection would make this test more straightforward
> > because you can explicitly control when I/Os fail and there's no need to
> > play around with dm-thin. That of course doesn't mean there isn't value
> > in having a test for fs' in dm-thin out of space conditions in general.
> > :)
> 
> Well, this is doable, although it has a caveat.
> 
> To do this, we'd need to inject the error in the buffer during IO completion,
> for example in xfs_buf_ioend(). The problem though, is that we start to have IOs
> before the 'mp->m_errotag' is actually initialized, so, xfs_buf_ioend() would
> need to check for m_errortag initialization before calling XFS_TEST_ERROR().
> 
> Something like this:
> 
> 	bp->b_flags &= ~(XBF_READ | XBF_WRITE | XBF_READ_AHEAD);
>  
> +	if (bp->b_target->bt_mount->m_errortag) {
> +		if (XFS_TEST_ERROR(false, bp->b_target->bt_mount,
> +				   XFS_ERRTAG_BUF_WB)) {
> +			bp->b_io_error = -ENOSPC;
> +		}
> +	}
> 
> This check could also be done in xfs_errortag_test(), although I'm not sure if
> it's worth, giving that there are very few places where error injection can be
> useful and m_errortag can be uninitialized.
> 
> 
> Thoughts?
> 

This strikes me as a bug in the error injection bits. Are you observing
a crash due to the injection point above? We should be able to add
injection points anywhere without such issues.

IOW, I think your suggestion to check ->m_errortag in xfs_test_error()
is probably the appropriate fix. Darrick may want to chime in.. but in
the meantime I'd suggest to throw up a patch to fix that. ;)

BTW, you may want to consider using somewhere like
xfs_buf_iodone_callbacks() as an independent injection point from
xfs_buf_ioend(). It seems the latter may potentially cause other I/O
errors that interfere with testing AIL writeback error processing. I
could be wrong though so it doesn't hurt to try (and we could certainly
add two separate injection points). Just something to think about..

Brian

> -- 
> Carlos
> --
> To unsubscribe from this list: send the line "unsubscribe linux-xfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html