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

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Date:   Thu, 31 Oct 2019 09:17:58 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     Boaz Harrosh <boaz@...xistor.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 Mon, Oct 28, 2019 at 09:10:39AM +1100, Dave Chinner wrote:
> On Fri, Oct 25, 2019 at 01:49:26PM -0700, Ira Weiny wrote:

[snip]

> 
> > Currently this works if I remount the fs or if I use <procfs>/drop_caches like
> > Boaz mentioned.
> 
> drop_caches frees all the dentries that don't have an active
> references before it iterates over inodes, thereby dropping the
> cached reference(s) to the inode that pins it in memory before it
> iterates the inode LRU.
> 
> > Isn't there a way to get xfs to do that on it's own?
> 
> Not reliably. Killing all the dentries doesn't guarantee the inode
> will be reclaimed immediately. The ioctl() itself requires an open
> file reference to the inode, and there's no telling how many other
> references there are to the inode that the filesystem a) can't find,
> and b) even if it can find them, it is illegal to release them.
> 
> IOWs, if you are relying on being able to force eviction of inode
> from the cache for correct operation of a user controlled flag, then
> it's just not going to work.

Agree, I see the difficulty of forcing the effective flag to change in this
path.  However, the only thing I am relying on is that the ioctl will change
the physical flag.

IOW I am proposing that the semantic be that changing the physical flag does
_not_ immediately change the effective flag.  With that clarified up front the
user can adjust accordingly.

After thinking about this more I think there is a strong use case to be able to
change the physical flag on a non-zero length file.  That use case is to be
able to restore files from backups.

Therefore, having the effective flag flip at some later time when the a_ops can
safely be changed (for example a remount/drop_cache event) is beneficial.

I propose the user has no direct control over this event and it is mainly used
to restore files from backups which is mainly an admin operation where a
remount is a reasonable thing to do.

Users direct control of the effective flag is through inheritance.  The user
needs to create the file in a DAX enable dir and they get effective operation
right away.

If in the future we can determine a safe way to trigger the a_ops change we can
add that to the semantic as an alternative for users.

Ira

> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@...morbit.com