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

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Message-ID: <76ea04b4-bad7-8cb3-d2c6-4ad49def4e05@fujitsu.com>
Date:   Wed, 14 Sep 2022 17:38:02 +0800
From:   Yang, Xiao/杨 晓 <yangx.jy@...itsu.com>
To:     Brian Foster <bfoster@...hat.com>,
        Ruan, Shiyang/阮 世阳 
        <ruansy.fnst@...itsu.com>
CC:     "Darrick J. Wong" <djwong@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "david@...morbit.com" <david@...morbit.com>,
        "hch@...radead.org" <hch@...radead.org>
Subject: Re: [PATCH] xfs: fail dax mount if reflink is enabled on a partition

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

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?

PS: I want to reproduce the issue and try to find a better solution to 
fix it.

Best Regards,
Xiao Yang

> 
> BTW, only log-writes, stripe and linear support DAX for now.