On Fri, Jun 30, 2017 at 07:33:42AM -0400, Brian Foster wrote:
> 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.
>
Yup, when calling XFS_TEST_ERROR() from xfs_buf_ioend(), this will cause a NULL
pointer dereference right when the filesystem is mounted. I didn't really got
deeper into it to know from where exactly it comes from, but, I'd guess it comes
from the very beginning, in xfs_fs_fill_super(), reading blocks from disk, will
end up triggering xfs_buf_ioend().
> 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. ;)
>
Sounds, reasonable, I'll write the patch and send in the next minutes.
> 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..
>
Thanks, I'll think about it.
Cheers
--
Carlos
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