linux-ext4 - Re: [PATCH 0/9] add ext4 per-inode DAX flag

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Date:   Mon, 11 Sep 2017 10:47:33 +0200
From:   Jan Kara <jack@...e.cz>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Dave Chinner <david@...morbit.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andreas Dilger <adilger@...ger.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Eric Sandeen <sandeen@...hat.com>,
        Lukas Czerner <lczerner@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@....de>, Jan Kara <jack@...e.cz>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        xfs <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH 0/9] add ext4 per-inode DAX flag

On Fri 08-09-17 11:39:13, Ted Tso wrote:
> On Fri, Sep 08, 2017 at 09:25:43AM +1000, Dave Chinner wrote:
> > > Okay, so other ideas (which you have also probably already though of) include:
> > > 
> > > 1) Just return -EBUSY if anyone tries to change the DAX flag of an inode with
> > > open mappings or any open file handles.
> > 
> > You have to have an open fd to change the flag. :)
> 
> What if we only allow the S_DAX flag to be *set*, when i_size and
> i_blocks is zero?  We could also require that only one file descriptor
> be open against the inode, and that it be opened O_RDONLY.

We could do something like that but IMHO it will be a pain to use (e.g.
think how difficult it would be to switch your existing database to use DAX
for data files). We can make transition reliable whenever
inode->i_mapping->i_mmap RB tree is empty (effectively: whenever the file
is not mmaped). And that should be relaxed enough for most usecases... But
I agree that it will be somewhat tricky to prevent creation of new mappings
while we are switching S_DAX flag so it needs more though.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR