linux-kernel - Re: [PATCH 2/2] dm log writes: add support for DAX

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Message-ID: <20171023173408.z6gog67bdp5s7jzy@destiny>
Date:   Mon, 23 Oct 2017 13:34:09 -0400
From:   Josef Bacik <josef@...icpanda.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, Josef Bacik <jbacik@...com>,
        Alasdair Kergon <agk@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Mike Snitzer <snitzer@...hat.com>,
        Shaohua Li <shli@...nel.org>, dm-devel@...hat.com,
        linux-nvdimm@...ts.01.org, linux-raid@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 2/2] dm log writes: add support for DAX

On Thu, Oct 19, 2017 at 11:24:04PM -0600, Ross Zwisler wrote:
> Now that we have the ability log filesystem writes using a flat buffer, add
> support for DAX.  Unfortunately we can't easily track data that has been
> written via mmap() now that the dax_flush() abstraction was removed by this
> commit:
> 
> commit c3ca015fab6d ("dax: remove the pmem_dax_ops->flush abstraction")
> 
> Otherwise we could just treat each flush as a big write, and store the data
> that is being synced to media.  It may be worthwhile to add the dax_flush()
> entry point back, just as a notifier so we can do this logging.
> 
> The motivation for this support is the need for an xfstest that can test
> the new MAP_SYNC DAX flag.  By logging the filesystem activity with
> dm-log-writes we can show that the MAP_SYNC page faults are writing out
> their metadata as they happen, instead of requiring an explicit
> msync/fsync.
> 
> Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> ---

Ok this is just my ignorance of how DAX works shining through, but do we need a
new flag to indicate this is DAX data?  You are logging it like it's just normal
data going to a certain sector, is that good enough?  If it is then hooray this
looks fine to me, I'm just slightly confused.  Thanks,

Josef