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

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Message-ID: <ce055127-5814-5335-43d2-4d0425425ba9@linux.alibaba.com>
Date:   Wed, 27 Oct 2021 10:52:42 +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: Re: [Question] ext4/xfs: Default behavior changed after per-file DAX

Sorry for the noise. Ira Weiny had replied my previous mail [1] but
unfortunately the reply was moved into junk box, and I didn't note that.

[1]
https://lore.kernel.org/all/YW2Oj4FrIB8do3zX@redhat.com/T/#mb022959fe3b6e9b82e2b066dd8daa301cd2b2c53

On 10/26/21 10:12 PM, JeffleXu wrote:
> 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