linux-kernel - Re: [PATCH v7 4/6] dax: add DAX_RECOVERY flag and .recovery_write dev_pgmap

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Message-ID: <CAPcyv4gfF4AhxD_vqCS9CTRraj8GAMDYQ7Zb411+FvxhF4ccOw@mail.gmail.com>
Date:   Mon, 11 Apr 2022 16:55:58 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jane Chu <jane.chu@...cle.com>,
        "david@...morbit.com" <david@...morbit.com>,
        "djwong@...nel.org" <djwong@...nel.org>,
        "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>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v7 4/6] dax: add DAX_RECOVERY flag and .recovery_write dev_pgmap_ops

On Wed, Apr 6, 2022 at 10:31 PM Christoph Hellwig <hch@...radead.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.