linux-ext4 - Re: [RFC PATCH 0/7] dax, ext4: Synchronous page faults

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Message-ID: <20170801110241.GE6742@infradead.org>
Date:   Tue, 1 Aug 2017 04:02:41 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Jeff Moyer <jmoyer@...hat.com>, linux-xfs@...r.kernel.org,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@...radead.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [RFC PATCH 0/7] dax, ext4: Synchronous page faults

On Fri, Jul 28, 2017 at 11:38:21AM +0200, Jan Kara wrote:
> Well, you are right I can make the implementation work with struct file
> flag as well - let's call it O_DAXDSYNC. However there are filesystem
> operations where you may need to answer question: Is there any fd with
> O_DAXDSYNC open against this inode (for operations that change file offset
> -> block mapping)? And in that case inode flag is straightforward while
> file flag is a bit awkward (you need to implement counter of fd's with that
> flag in the inode).

We can still keep and inode flag as the internal implementation
detail.  As mentioned earlier the right flag to control behavior
of a mapping is an mmap flag.  And the initial naive implementation
would simply mark the inode as sync once the first MAP_SYNC open happens
on it.  We could then move to more precise tracking if/when needed.