linux-ext4 - [Question] ext4/xfs: Default behavior changed after per-file DAX

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Date:   Tue, 26 Oct 2021 22:12:17 +0800
From:   JeffleXu <jefflexu@...ux.alibaba.com>
To:     Theodore Ts'o <tytso@....edu>, adilger.kernel@...ger.ca,
        "Darrick J. Wong" <djwong@...nel.org>, ira.weiny@...el.com
Cc:     linux-xfs@...r.kernel.org,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, dan.j.williams@...el.com,
        Vivek Goyal <vgoyal@...hat.com>, Christoph Hellwig <hch@....de>
Subject: [Question] ext4/xfs: Default behavior changed after per-file DAX

Hi,

Recently I'm working on supporting per-file DAX for virtiofs [1]. Vivek
Goyal and I are interested [2] why the default behavior has changed
since introduction of per-file DAX on ext4 and xfs [3][4].

That is, before the introduction of per-file DAX, when user doesn't
specify '-o dax', DAX is disabled for all files. After supporting
per-file DAX, when neither '-o dax' nor '-o dax=always|inode|never' is
specified, it actually works in a '-o dax=inode' way if the underlying
blkdev is DAX capable, i.e. depending on the persistent inode flag. That
is, the default behavior has changed from user's perspective.

We are not sure if this is intentional or not. Appreciate if anyone
could offer some hint.

[1] https://lore.kernel.org/all/YW2Oj4FrIB8do3zX@redhat.com/T/
[2]
https://lore.kernel.org/all/YW2Oj4FrIB8do3zX@redhat.com/T/#mf067498887ca2023c64c8b8f6aec879557eb28f8
[3] 9cb20f94afcd2964944f9468e38da736ee855b19 ("fs/ext4: Make DAX mount
option a tri-state")
[4] 02beb2686ff964884756c581d513e103542dcc6a ("fs/xfs: Make DAX mount
option a tri-state")

-- 
Thanks,
Jeffle