[Virtio-fs] xfstest generic/503 hangs

[Max Reitz]

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.

Max

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