linux-kernel - Re: [dm-devel] [PATCH 0/6] dax poison recovery with RWF_RECOVERY

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Message-ID: <20211022015817.GY24307@magnolia>
Date:   Thu, 21 Oct 2021 18:58:17 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Jane Chu <jane.chu@...cle.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "david@...morbit.com" <david@...morbit.com>,
        "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: [dm-devel] [PATCH 0/6] dax poison recovery with
 RWF_RECOVERY_DATA flag

On Fri, Oct 22, 2021 at 01:37:28AM +0000, Jane Chu wrote:
> On 10/21/2021 4:31 AM, Christoph Hellwig wrote:
> > Looking over the series I have serious doubts that overloading the
> > slow path clear poison operation over the fast path read/write
> > path is such a great idea.

Why would data recovery after a media error ever be considered a
fast/hot path?  A normal read/write to a fsdax file would not pass the
flag, which skips the poison checking with whatever MCE consequences
that has, right?

pwritev2(..., RWF_RECOVER_DATA) should be infrequent enough that
carefully stepping around dax_direct_access only has to be faster than
restoring from backup, I hope?

> Understood, sounds like a concern on principle. But it seems to me
> that the task of recovery overlaps with the normal write operation
> on the write part. Without overloading some write operation for
> 'recovery', I guess we'll need to come up with a new userland
> command coupled with a new dax API ->clear_poison and propagate the
> new API support to each dm targets that support dax which, again,
> is an idea that sounds too bulky if I recall Dan's earlier rejection
> correctly.
> 
> It is in my plan though, to provide pwritev2(2) and preadv2(2) man pages
> with description about the RWF_RECOVERY_DATA flag and specifically not
> recommending the flag for normal read/write operation - due to potential
> performance impact from searching badblocks in the range.

Yes, this will help much. :)

> That said, the badblock searching is turned on only if the pmem device
> contains badblocks(i.e. bb->count > 0), otherwise, the performance
> impact is next to none. And once the badblock search starts,
> it is a binary search over user provided range. The unwanted impact
> happens to be the case when the pmem device contains badblocks
> that do not fall in the user specified range, and in that case, the
> search would end in O(1).

(I wonder about improving badblocks to be less sector-oriented and not
have that weird 16-records-max limit, but that can be a later
optimization.)

--D

> Thanks!
> -jane
> 
>