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

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Message-ID: <847e8c8c-f1b0-66f5-f3e6-e2f6c618a80f@oracle.com>
Date:   Thu, 14 Apr 2022 00:48:16 +0000
From:   Jane Chu <jane.chu@...cle.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Christoph Hellwig <hch@...radead.org>
CC:     "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 4/11/2022 4:55 PM, Dan Williams wrote:
> 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.

Okay, will do in next revision.

thanks!
-jane