[Virtio-fs] xfstest generic/503 hangs

[Dr. David Alan Gilbert]

* Max Reitz (mreitz at redhat.com) wrote:
> Hi,
> 
> I have this bug report here:
> https://bugzilla.redhat.com/show_bug.cgi?id=1813885
> 
> And I’m afraid I’m not really making progress on debugging it, so I was
> wondering whether any of you might have some insights.
> 
> The problem is that the generic/503 xfstest hangs on virtio-fs.  Now, I
> don’t know how the reporter got that test to run in the first place,
> because for me, it requires fcollapse and fzero, which as far as I can
> tell are currently not supported for virtio-fs.
> 
> So I first had to disable those requirements, and then let the helper
> program (src/t_mmap_collision.c) not test those operations.
> 
> Then, the test hangs.  What I could find out so far is that the hang
> occurs in src/t_mmap_collision.c’s truncate_down_fn() (run through
> run_test(&truncate_down_fn), namely in one of the pread()s.  I can also
> see that some of the pread()s before fail with EFAULT.
> 
> A bit more context: t_mmap_collision.c opens a test file twice (I think
> the idea is that you open it once on an FS with DAX, and once without,
> but AFAIU it should work either way).  For the relevant test, it mmap()s
> the DAX FD, truncates it, then fallocates it to increase the size again.
>  Then it reads from the non-DAX FD.

Can you just confirm where the DAX is happening here?  As I read that bz
entry it's using the qemu which doesn't have DAX code yet.

Dave

> It does all of that in two threads simultaneously for a second.
> 
> The EFAULT seems to come from the guest kernel.  I don’t see virtiofsd
> returning an error anywhere.  I don’t know where it comes from exactly,
> only that when I replace all occurrences of “EFAULT” by e.g. “EBADSLT”
> in mm/, the test crashes instead of hanging, so I take that to mean that
> the error comes from something in mm/ (which I suppose isn’t too
> unexpected).
> 
> The test passes if running the test function in a single thread instead
> of two, or if you use a separate TEST_DEV and SCRATCH_DEV – but in the
> latter case, you really have two separate files, so the test becomes
> rather moot (AFAIU).
> 
> The fact that truncate_down_fn() uses fallocate() seems irrelevant.
> When you replace it by ftruncate() (i.e. the dax_fd is just first
> truncated to 0, and then truncated back to @file_size), the test fails
> in the same way.  So maybe there is some interaction between the
> ftruncate() and a concurrent pread()?  But where does the EFAULT come from?
> 
> Does anyone have any spontaneous ideas? :/
> 
> 
> In any case, thanks already for reading this,
> 
> Max
> 
> 
> (I suppose my plan now is that instead of debugging the kernel further,
> I should come up with a simpler reproducer, to see whether the problem
> is really just a concurrent ftruncate() + pread() on two FDs that point
> to the same file.)
> 




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--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK