linux-kernel - Re: [PATCH 4/6] dm,dax,pmem: prepare dax_copy_to/from_iter() APIs with DAXDEV_F

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Date:   Thu, 21 Oct 2021 22:33:33 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jane Chu <jane.chu@...cle.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "david@...morbit.com" <david@...morbit.com>,
        "djwong@...nel.org" <djwong@...nel.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "vishal.l.verma@...el.com" <vishal.l.verma@...el.com>,
        "dave.jiang@...el.com" <dave.jiang@...el.com>,
        "agk@...hat.com" <agk@...hat.com>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        "ira.weiny@...el.com" <ira.weiny@...el.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "vgoyal@...hat.com" <vgoyal@...hat.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>
Subject: Re: [PATCH 4/6] dm,dax,pmem: prepare dax_copy_to/from_iter() APIs
 with DAXDEV_F_RECOVERY

On Fri, Oct 22, 2021 at 12:49:15AM +0000, Jane Chu wrote:
> I've looked through your "futher decouple DAX from block devices" series 
> and likes the use of xarray in place of the host hash list.
> Which upstream version is the series based upon?
> If it's based on your development repo, I'd be happy to take a clone
> and rebase my patches on yours if you provide a link. Please let me
> know the best way to cooperate.

It is based on linux-next from when it was posted.  A git tree is here:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dax-block-cleanup

> That said, I'm unclear at what you're trying to suggest with respect
> to the 'DAXDEV_F_RECOVERY' flag.  The flag came from upper dax-fs
> call stack to the dm target layer, and the dm targets are equipped
> with handling pmem driver specific task, so it appears that the flag 
> would need to be passed down to the native pmem layer, right?
> Am I totally missing your point?

We'll need to pass it through (assuming we want to keep supporting
dm, see the recent discussion with Dan).

FYI, here is a sketch where I'd like to move to, but this isn't properly
tested yet:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dax-devirtualize

To support something like DAXDEV_F_RECOVERYwe'd need a separate
dax_operations methods.  Which to me suggest it probably should be
a different operation (fallocate / ioctl / etc) as Darrick did earlier.