Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
MIME-Version: 1.0
References: <20220405194747.2386619-1-jane.chu@oracle.com> <20220405194747.2386619-5-jane.chu@oracle.com>
 <Yk0i/pODntZ7lbDo@infradead.org> <196d51a3-b3cc-02ae-0d7d-ee6fbb4d50e4@oracle.com>
 <Yk52415cnFa39qil@infradead.org>
In-Reply-To: <Yk52415cnFa39qil@infradead.org>
From:   Dan Williams <dan.j.williams@intel.com>
Date:   Mon, 11 Apr 2022 16:55:58 -0700
Message-ID: <CAPcyv4gfF4AhxD_vqCS9CTRraj8GAMDYQ7Zb411+FvxhF4ccOw@mail.gmail.com>
Subject: Re: [PATCH v7 4/6] dax: add DAX_RECOVERY flag and .recovery_write dev_pgmap_ops
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Jane Chu <jane.chu@oracle.com>,
        "david@fromorbit.com" <david@fromorbit.com>,
        "djwong@kernel.org" <djwong@kernel.org>,
        "vishal.l.verma@intel.com" <vishal.l.verma@intel.com>,
        "dave.jiang@intel.com" <dave.jiang@intel.com>,
        "agk@redhat.com" <agk@redhat.com>,
        "snitzer@redhat.com" <snitzer@redhat.com>,
        "dm-devel@redhat.com" <dm-devel@redhat.com>,
        "ira.weiny@intel.com" <ira.weiny@intel.com>,
        "willy@infradead.org" <willy@infradead.org>,
        "vgoyal@redhat.com" <vgoyal@redhat.com>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "nvdimm@lists.linux.dev" <nvdimm@lists.linux.dev>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-xfs@vger.kernel.org" <linux-xfs@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, Apr 6, 2022 at 10:31 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Wed, Apr 06, 2022 at 05:32:31PM +0000, Jane Chu wrote:
> > Yes, I believe Dan was motivated by avoiding the dm dance as a result of
> > adding .recovery_write to dax_operations.
> >
> > I understand your point about .recovery_write is device specific and
> > thus not something appropriate for device agnostic ops.
> >
> > I can see 2 options so far -
> >
> > 1)  add .recovery_write to dax_operations and do the dm dance to hunt
> > down to the base device that actually provides the recovery action
>
> That would be my preference.  But I'll wait for Dan to chime in.

Yeah, so the motivation was avoiding plumbing recovery through stacked
lookups when the recovery is specific to a pfn and the provider of
that pfn, but I also see it from Christoph's perspective that the only
agent that cares about recovery is the fsdax I/O path. Certainly
having ->dax_direct_access() take a DAX_RECOVERY flag and the op
itself go through the pgmap is a confusing split that I did not
anticipate when I made the suggestion. Since that flag must be there,
then the ->recovery_write() should also stay relative to a dax device.

Apologies for the thrash Jane.

One ask though, please separate plumbing the new flag argument to
->dax_direct_access() and plumbing the new operation into preparation
patches before filling them in with the new goodness.