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

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Message-ID: <7fdc9e88-f255-6edb-7964-a5a82e9b1292@fujitsu.com>
Date:   Wed, 14 Sep 2022 14:44:08 +0800
From:   Yang, Xiao/杨 晓 <yangx.jy@...itsu.com>
To:     Brian Foster <bfoster@...hat.com>,
        Shiyang Ruan <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 2022/9/9 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?

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

Best Regards,
Xiao Yang