Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <YXFPfEGjoUaajjL4@infradead.org> <e89a2b17-3f03-a43e-e0b9-5d2693c3b089@oracle.com>
 <YXJN4s1HC/Y+KKg1@infradead.org> <2102a2e6-c543-2557-28a2-8b0bdc470855@oracle.com>
 <YXj2lwrxRxHdr4hb@infradead.org> <20211028002451.GB2237511@magnolia>
 <YYDYUCCiEPXhZEw0@infradead.org> <CAPcyv4j8snuGpy=z6BAXogQkP5HmTbqzd6e22qyERoNBvFKROw@mail.gmail.com>
 <YYK/tGfpG0CnVIO4@infradead.org> <CAPcyv4it2_PVaM8z216AXm6+h93frg79WM-ziS9To59UtEQJTA@mail.gmail.com>
 <YYOaOBKgFQYzT/s/@infradead.org> <CAPcyv4jKHH7H+PmcsGDxsWA5CS_U3USHM8cT1MhoLk72fa9z8Q@mail.gmail.com>
 <6d21ece1-0201-54f2-ec5a-ae2f873d46a3@oracle.com> <CAPcyv4hJjcy2TnOv-Y5=MUMHeDdN-BCH4d0xC-pFGcHXEU_ZEw@mail.gmail.com>
 <342eb71c-0aff-77e5-3c71-92224d7d48e0@oracle.com>
In-Reply-To: <342eb71c-0aff-77e5-3c71-92224d7d48e0@oracle.com>
From:   Dan Williams <dan.j.williams@intel.com>
Date:   Thu, 4 Nov 2021 17:46:17 -0700
Message-ID: <CAPcyv4hVu+A0PXgXTwWj3SBimP5pjX_97g+sfGeT47P0-SJkiQ@mail.gmail.com>
Subject: Re: [dm-devel] [PATCH 0/6] dax poison recovery with RWF_RECOVERY_DATA flag
To:     Jane Chu <jane.chu@oracle.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>,
        "david@fromorbit.com" <david@fromorbit.com>,
        "vishal.l.verma@intel.com" <vishal.l.verma@intel.com>,
        "dave.jiang@intel.com" <dave.jiang@intel.com>,
        "agk@redhat.com" <agk@redhat.com>,
        "snitzer@redhat.com" <snitzer@redhat.com>,
        "dm-devel@redhat.com" <dm-devel@redhat.com>,
        "ira.weiny@intel.com" <ira.weiny@intel.com>,
        "willy@infradead.org" <willy@infradead.org>,
        "vgoyal@redhat.com" <vgoyal@redhat.com>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "nvdimm@lists.linux.dev" <nvdimm@lists.linux.dev>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-xfs@vger.kernel.org" <linux-xfs@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Thu, Nov 4, 2021 at 1:27 PM Jane Chu <jane.chu@oracle.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.