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

[Gotou, Yasunori/五島 康文]

Hello everyone,

On 2022/09/20 11:38, Yang, Xiao/杨 晓 wrote:
> 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@xxxxxx/
>
> 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?


Though I asked other place(*), I really want to know the usecase of
dm-thin-volume with DAX and reflink.


In my understanding, dm-thin-volume seems to provide similar feature 
like reflink of xfs. Both feature provide COW update to reduce usage of
its region, and snapshot feature, right?

I found that docker seems to select one of them (or other feature which 
supports COW). Then user don't need to use thin-volume and reflink at 
same time.

Database which uses FS-DAX may want to use snapshot for its data of 
FS-DAX, its user seems to be satisfied with reflink or thin-volume.

So I could not find on what use-case user would like to use 
dm-thin-volume and reflink at same time.

The only possibility is that the user has mistakenly configured 
dm-thinpool and reflink to be used at the same time, but if that is the 
case, it seems to be better for the user to disable one or the other.

I really wander why dm-thin-volume must be used with reflik and FS-DAX.

If my understanding is something wrong, please correct me.

(*)https://lore.kernel.org/all/TYWPR01MB1008258F474CA2295B4CD3D9B90549@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

Thanks,
---
Yasunori Goto


> 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@xxxxxxxxxxx/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@xxxxxx/
> > >
> > > > > 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.