linux-ext4 - Re: [PATCH 2/7] dax: Add sync argument to dax_iomap

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Message-ID: <20170727220637.GC22000@linux.intel.com>
Date:   Thu, 27 Jul 2017 16:06:37 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andy Lutomirski <luto@...nel.org>, linux-nvdimm@...ts.01.org,
        linux-xfs@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 2/7] dax: Add sync argument to dax_iomap_fault()

On Thu, Jul 27, 2017 at 03:12:40PM +0200, Jan Kara wrote:
> Add 'sync' argument to dax_iomap_fault(). It will be used to communicate
> the fact that synchronous fault is requested.

I don't actually think you need to pass this 'sync' parameter around.  I think
you can completely rely on IOMAP_F_NEEDSYNC being set in iomap.flags.  The DAX
fault handlers can call ops->iomap_begin() and use that flag for all the
tests and make it our once source of truth.

That flag also tells us that we are doing a write fault (from
ext4_iomap_begin()):

	if ((flags & IOMAP_FAULT) && (flags & IOMAP_WRITE) && IS_SYNC(inode) &&
	    !jbd2_transaction_committed(EXT4_SB(inode->i_sb)->s_journal,
					EXT4_I(inode)->i_datasync_tid))
		iomap->flags |= IOMAP_F_NEEDDSYNC;

So conditionals like this from dax_iomap_pte_fault():

	force_ro = (vmf->flags & FAULT_FLAG_WRITE) && sync &&
			(iomap.flags & IOMAP_F_NEEDDSYNC);

can be simplified to:

	force_ro = (iomap.flags & IOMAP_F_NEEDDSYNC);