linux-kernel - Re: [PATCH v8 10/18] dax, dm: introduce ->fs_{claim, release}() dax

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Date:   Tue, 3 Apr 2018 15:39:13 -0400
From:   Mike Snitzer <snitzer@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Alasdair Kergon <agk@...hat.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Christoph Hellwig <hch@....de>, Jan Kara <jack@...e.cz>,
        david <david@...morbit.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 10/18] dax, dm: introduce ->fs_{claim, release}()
 dax_device infrastructure

On Tue, Apr 03 2018 at  2:24pm -0400,
Dan Williams <dan.j.williams@...el.com> wrote:

> On Fri, Mar 30, 2018 at 9:03 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> > In preparation for allowing filesystems to augment the dev_pagemap
> > associated with a dax_device, add an ->fs_claim() callback. The
> > ->fs_claim() callback is leveraged by the device-mapper dax
> > implementation to iterate all member devices in the map and repeat the
> > claim operation across the array.
> >
> > In order to resolve collisions between filesystem operations and DMA to
> > DAX mapped pages we need a callback when DMA completes. With a callback
> > we can hold off filesystem operations while DMA is in-flight and then
> > resume those operations when the last put_page() occurs on a DMA page.
> > The ->fs_claim() operation arranges for this callback to be registered,
> > although that implementation is saved for a later patch.
> >
> > Cc: Alasdair Kergon <agk@...hat.com>
> > Cc: Mike Snitzer <snitzer@...hat.com>
> 
> Mike, do these DM touches look ok to you?  We need these ->fs_claim()
> / ->fs_release() interfaces for device-mapper to set up filesystem-dax
> infrastructure on all sub-devices whenever a dax-capable DM device is
> mounted. It builds on the device-mapper dax dependency removal
> patches.

I'd prefer dm_dax_iterate() be renamed to dm_dax_iterate_devices()

But dm_dax_iterate() is weird... it is simply returning the struct
dax_device *dax_dev that is passed: seemingly without actually directly
changing anything about that dax_device (I can infer that you're
claiming the underlying devices, but...)

In general user's of ti->type->iterate_devices can get a result back
(via 'int' return).. you aren't using it that way (and maybe dax will
never have a need to return an answer).  But all said, I think I'd
prefer to see dm_dax_iterate_devices() return void.

But please let me know if I'm missing something, thanks.

Mike