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Date:   Fri, 16 Sep 2022 10:04:17 +0800
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Subject: Re: [PATCH] xfs: fail dax mount if reflink is enabled on a partition
From:   =?UTF-8?B?WWFuZywgWGlhby/mnagg5pmT?= <yangx.jy@fujitsu.com>
To:     "Darrick J. Wong" <djwong@kernel.org>,
        Brian Foster <bfoster@redhat.com>,
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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.
>>>>
>>>