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

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Date:   Thu, 4 Nov 2021 17:46:17 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jane Chu <jane.chu@...cle.com>
Cc:     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 4, 2021 at 1:27 PM Jane Chu <jane.chu@...cle.com> wrote:
>
> On 11/4/2021 12:00 PM, Dan Williams wrote:
>
> >>
> >> If this understanding is in the right direction, then I'd like to
> >> propose below changes to
> >>     dax_direct_access(), dax_copy_to/from_iter(), pmem_copy_to/from_iter()
> >>     and the dm layer copy_to/from_iter, dax_iomap_iter().
> >>
> >> 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".
> >
> >> 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/
>
> I was referring to
>
> http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dax-devirtualize
> that has not come out yet, I said early on that I'll rebase on it,
> but looks like we still need pmem_copy_to/from_iter(), so.

Yeah, since the block-layer divorce gets rid of the old poison
clearing path, then we're back to pmem_copy_to_iter() (or something
like it) needing to pick up the slack for poison clearing. I do agree
it would be nice to clean up all the unnecessary boilerplate, but the
error-list coordination requires a driver specific callback. At least
the DAX_F_VIRTUAL flag can eliminate the virtiofs and fuse callbacks.