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

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Date:   Thu, 4 Nov 2021 22:56:48 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jane Chu <jane.chu@...cle.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        "david@...morbit.com" <david@...morbit.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 Thu, Nov 04, 2021 at 12:00:12PM -0700, Dan Williams wrote:
> > 1. dax_iomap_iter() rely on dax_direct_access() to decide whether there
> >     is likely media error: if the API without DAX_F_RECOVERY returns
> >     -EIO, then switch to recovery-read/write code.  In recovery code,
> >     supply DAX_F_RECOVERY to dax_direct_access() in order to obtain
> >     'kaddr', and then call dax_copy_to/from_iter() with DAX_F_RECOVERY.
> 
> I like it. It allows for an atomic write+clear implementation on
> capable platforms and coordinates with potentially unmapped pages. The
> best of both worlds from the dax_clear_poison() proposal and my "take
> a fault and do a slow-path copy".

Fine with me as well.

> 
> > 2. the _copy_to/from_iter implementation would be largely the same
> >     as in my recent patch, but some changes in Christoph's
> >     'dax-devirtualize' maybe kept, such as DAX_F_VIRTUAL, obviously
> >     virtual devices don't have the ability to clear poison, so no need
> >     to complicate them.  And this also means that not every endpoint
> >     dax device has to provide dax_op.copy_to/from_iter, they may use the
> >     default.
> 
> Did I miss this series or are you talking about this one?
> https://lore.kernel.org/all/20211018044054.1779424-1-hch@lst.de/

Yes.  This is an early RFC, but I plan to finish this up and submit
it after the updated decouple series. 

> 
> > I'm not sure about nova and others, if they use different 'write' other
> > than via iomap, does that mean there will be need for a new set of
> > dax_op for their read/write?
> 
> No, they're out-of-tree they'll adjust to the same interface that xfs
> and ext4 are using when/if they go upstream.

Yepp.