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

[Dan Williams <dan.j.williams@intel.com>]

On Tue, Apr 3, 2018 at 12:39 PM, Mike Snitzer <snitzer@redhat.com> wrote:
> On Tue, Apr 03 2018 at  2:24pm -0400,
> Dan Williams <dan.j.williams@intel.com> wrote:
>
>> On Fri, Mar 30, 2018 at 9:03 PM, Dan Williams <dan.j.williams@intel.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@redhat.com>
>> > Cc: Mike Snitzer <snitzer@redhat.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()

Ok, I'll fix that up.

> 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...)

I could at least add a note to see the comment in dm_dax_dev_claim().
The filesystem caller expects to get a dax_dev back or NULL from
fs_dax_claim_bdev() if the claim failed. For fs_dax_claim() the return
value could simply be bool for pass / fail, but I used dax_dev NULL /
not-NULL instead.

In the case of device-mapper the claim attempt can't fail for
conflicting ownership reasons because the exclusive ownership of the
underlying block device is already established by device-mapper before
the fs claims the device-mapper dax device.

> 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.

Oh, yeah, I like that better. I'll just make it return void and have
dm_dax_fs_claim() return the dax_dev directly.

Thanks Mike!
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