Re: [PATCH] xfs: fail dax mount if reflink is enabled on a partition

Yang, Xiao/杨 晓
2022-09-20T02:39:08Z

Hi Darrick, Brian and Christoph

Ping. I hope to get your feedback.

1) I have confirmed that the following patch set did not change the test
result of generic/470 with thin-volume. Besides, I didn't see any
failure when running generic/470 based on normal PMEM device instaed of
thin-volume.
https://lore.kernel.org/linux-xfs/20211129102203.2243509-1-hch@lst.de/

2) I can reproduce the failure of generic/482 without thin-volume.

3) Is it necessary to make thin-volume support DAX. Is there any use
case for the requirement?

Best Regards,
Xiao Yang

On 2022/9/16 10:04, Yang, Xiao/杨 晓 wrote:
> On 2022/9/15 18:14, Yang, Xiao/杨 晓 wrote:
>> On 2022/9/15 0:28, Darrick J. Wong wrote:
>>> On Wed, Sep 14, 2022 at 08:34:26AM -0400, Brian Foster wrote:
>>>> On Wed, Sep 14, 2022 at 05:38:02PM +0800, Yang, Xiao/杨 晓 wrote:
>>>>> On 2022/9/14 14:44, Yang, Xiao/杨 晓 wrote:
>>>>>> On 2022/9/9 21:01, Brian Foster wrote:
>>>>>>> Yes.. I don't recall all the internals of the tools and test, but
>>>>>>> IIRC
>>>>>>> it relied on discard to perform zeroing between checkpoints or
>>>>>>> some such
>>>>>>> and avoid spurious failures. The purpose of running on dm-thin was
>>>>>>> merely to provide reliable discard zeroing behavior on the target
>>>>>>> device
>>>>>>> and thus to allow the test to run reliably.
>>>>>> Hi Brian,
>>>>>>
>>>>>> As far as I know, generic/470 was original designed to verify
>>>>>> mmap(MAP_SYNC) on the dm-log-writes device enabling DAX. Due to the
>>>>>> reason, we need to ensure that all underlying devices under
>>>>>> dm-log-writes device support DAX. However dm-thin device never
>>>>>> supports
>>>>>> DAX so
>>>>>> running generic/470 with dm-thin device always returns "not run".
>>>>>>
>>>>>> Please see the difference between old and new logic:
>>>>>>
>>>>>>             old logic                          new logic
>>>>>> ---------------------------------------------------------------
>>>>>> log-writes device(DAX)                 log-writes device(DAX)
>>>>>>               |                                       |
>>>>>> PMEM0(DAX) + PMEM1(DAX)       Thin device(non-DAX) + PMEM1(DAX)
>>>>>>                                             |
>>>>>>                                           PMEM0(DAX)
>>>>>> ---------------------------------------------------------------
>>>>>>
>>>>>> We think dm-thin device is not a good solution for generic/470, is
>>>>>> there
>>>>>> any other solution to support both discard zero and DAX?
>>>>>
>>>>> Hi Brian,
>>>>>
>>>>> I have sent a patch[1] to revert your fix because I think it's not
>>>>> good for
>>>>> generic/470 to use thin volume as my revert patch[1] describes:
>>>>> [1]
>>>>> https://lore.kernel.org/fstests/20220914090625.32207-1-yangx.jy@fujitsu.com/T/#u
>>>>>
>>>>>
>>>>
>>>> I think the history here is that generic/482 was changed over first in
>>>> commit 65cc9a235919 ("generic/482: use thin volume as data device"),
>>>> and
>>>> then sometime later we realized generic/455,457,470 had the same
>>>> general
>>>> flaw and were switched over. The dm/dax compatibility thing was
>>>> probably
>>>> just an oversight, but I am a little curious about that because it
>>>> should
>>>
>>> It's not an oversight -- it used to work (albeit with EXPERIMENTAL
>>> tags), and now we've broken it on fsdax as the pmem/blockdev divorce
>>> progresses.
>> Hi
>>
>> Do you mean that the following patch set changed the test result of
>> generic/470 with thin-volume? (pass => not run/failure)
>> https://lore.kernel.org/linux-xfs/20211129102203.2243509-1-hch@lst.de/
>>
>>>
>>>> have been obvious that the change caused the test to no longer run. Did
>>>> something change after that to trigger that change in behavior?
>>>>
>>>>> With the revert, generic/470 can always run successfully on my
>>>>> environment
>>>>> so I wonder how to reproduce the out-of-order replay issue on XFS v5
>>>>> filesystem?
>>>>>
>>>>
>>>> I don't quite recall the characteristics of the failures beyond that we
>>>> were seeing spurious test failures with generic/482 that were due to
>>>> essentially putting the fs/log back in time in a way that wasn't quite
>>>> accurate due to the clearing by the logwrites tool not taking place. If
>>>> you wanted to reproduce in order to revisit that, perhaps start with
>>>> generic/482 and let it run in a loop for a while and see if it
>>>> eventually triggers a failure/corruption..?
>>>>
>>>>> PS: I want to reproduce the issue and try to find a better solution
>>>>> to fix
>>>>> it.
>>>>>
>>>>
>>>> It's been a while since I looked at any of this tooling to semi-grok
>>>> how
>>>> it works.
>>>
>>> I /think/ this was the crux of the problem, back in 2019?
>>> https://lore.kernel.org/fstests/20190227061529.GF16436@dastard/
>>
>> Agreed.
>>
>>>
>>>> Perhaps it could learn to rely on something more explicit like
>>>> zero range (instead of discard?) or fall back to manual zeroing?
>>>
>>> AFAICT src/log-writes/ actually /can/ do zeroing, but (a) it probably
>>> ought to be adapted to call BLKZEROOUT and (b) in the worst case it
>>> writes zeroes to the entire device, which is/can be slow.
>>>
>>> For a (crass) example, one of my cloudy test VMs uses 34GB partitions,
>>> and for cost optimization purposes we're only "paying" for the cheapest
>>> tier.  Weirdly that maps to an upper limit of 6500 write iops and
>>> 48MB/s(!) but that would take about 20 minutes to zero the entire
>>> device if the dm-thin hack wasn't in place.  Frustratingly, it doesn't
>>> support discard or write-zeroes.
>>
>> Do you mean that discard zero(BLKDISCARD) is faster than both fill
>> zero(BLKZEROOUT) and write zero on user space?
>
> Hi Darrick, Brian and Christoph
>
> According to the discussion about generic/470. I wonder if it is
> necessary to make thin-pool support DAX. Is there any use case for the
> requirement?
>
> Best Regards,
> Xiao Yang
>>
>> Best Regards,
>> Xiao Yang
>>>
>>>> If the
>>>> eventual solution is simple and low enough overhead, it might make some
>>>> sense to replace the dmthin hack across the set of tests mentioned
>>>> above.
>>>
>>> That said, for a *pmem* test you'd expect it to be faster than that...
>>>
>>> --D
>>>
>>>> Brian
>>>>
>>>>> Best Regards,
>>>>> Xiao Yang
>>>>>
>>>>>>
>>>>>> BTW, only log-writes, stripe and linear support DAX for now.
>>>>>
>>>>