linux-ext4 - Re: [PATCH 0/5] Enable per-file/directory DAX operations

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Date:   Fri, 25 Oct 2019 04:15:44 +0300
From:   Boaz Harrosh <boaz@...xistor.com>
To:     Dave Chinner <david@...morbit.com>,
        Boaz Harrosh <boaz@...xistor.com>
Cc:     ira.weiny@...el.com, linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/5] Enable per-file/directory DAX operations

On 25/10/2019 03:36, Dave Chinner wrote:
> On Fri, Oct 25, 2019 at 02:29:04AM +0300, Boaz Harrosh wrote:
<>

>> Perhaps we always go by the directory. And then do an mv dir_DAX/foo dir_NODAX/foo
> 
> The inode is instatiated before the rename is run, so it's set up
> with it's old dir config, not the new one. So this ends up with the
> same problem of haivng to change the S_DAX flag and aops vector
> dynamically on rename. Same problem, not a solution.
> 

Yes Admin needs a inode-drop_caches after the mv if she/he wants an effective
change.

>> to have an effective change. In hard links the first one at iget time before populating
>> the inode cache takes affect.
> 
> If something like a find or backup program brings the inode into
> cache, the app may not even get the behaviour it wants, and it can't
> change it until the inode is evicted from cache, which may be never.

inode-drop-caches. (echo 2 > /proc/sys/vm/drop_caches)

> Nobody wants implicit/random/uncontrollable/unchangeable behaviour
> like this.
> 

You mean in the case of hard links between different mode directories?
I agree it is not so good. I do not like it too.

<>
> We went over all this ground when we disabled the flag in the first
> place. We disabled the flag because we couldn't come up with a sane
> way to flip the ops vector short of tracking the number of aops
> calls in progress at any given time. i.e. reference counting the
> aops structure, but that's hard to do with a const ops structure,
> and so it got disabled rather than allowing users to crash
> kernels....
> 

Do you mean dropping this patchset all together? I missed that.

Current patchset with the i_size == 0 thing is really bad I think.
Its the same has dropping the direct change all together and only
supporting inheritance from parent.
[Which again for me is really not interesting]

> Cheers,
> -Dave.

Lets sleep on it. Please remind me if xfs supports clone + DAX

Thanks Dave
Boaz