[Virtio-fs] xfstest generic/503 hangs

[Max Reitz]

On 24.03.20 20:52, Dr. David Alan Gilbert wrote:
> * Max Reitz (mreitz at redhat.com) wrote:
>> On 23.03.20 19:43, Dr. David Alan Gilbert wrote:
>>> * 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.
>>
>> I actually don’t know whether it really uses DAX.  When I say “DAX” here
>> (and “dax_fd”), I mean it in the spirit of the test, which tries to open
>> a file once with DAX and once without.  But it isn’t like it verifies
>> that the first instance actually uses DAX.
>>
>> So I think it rather likely that it doesn’t use DAX, and just two FDs
>> for a single file concurrently.
> 
> OK, I don't think DAX is really involved.
> 
> Does Liu Bo's: 
> https://www.redhat.com/archives/virtio-fs/2020-March/msg00074.html
> 
> help?

No, unfortunately it doesn’t.

(I tried it last week, I was looking for patches that might be related;
the only other thing I tried was
https://www.redhat.com/archives/virtio-fs/2020-March/msg00002.html)

Max

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