linux-kernel - Re: [PATCH v3 01/11] pagemap: Introduce ->memory

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Date:   Wed, 24 Mar 2021 09:37:01 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     "ruansy.fnst@...itsu.com" <ruansy.fnst@...itsu.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        device-mapper development <dm-devel@...hat.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        david <david@...morbit.com>, Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Goldwyn Rodrigues <rgoldwyn@...e.de>,
        "qi.fuli@...itsu.com" <qi.fuli@...itsu.com>,
        "y-goto@...itsu.com" <y-goto@...itsu.com>
Subject: Re: [PATCH v3 01/11] pagemap: Introduce ->memory_failure()

On Wed, Mar 24, 2021 at 12:48 AM Christoph Hellwig <hch@....de> wrote:
>
> On Tue, Mar 23, 2021 at 07:19:28PM -0700, Dan Williams wrote:
> > So I think the path forward is:
> >
> > - teach memory_failure() to allow for ranged failures
> >
> > - let interested drivers register for memory failure events via a
> > blocking_notifier_head
>
> Eww.  As I said I think the right way is that the file system (or
> other consumer) can register a set of callbacks for opening the device.

How does that solve the problem of the driver being notified of all
pfn failure events? Today pmem only finds out about the ones that are
notified via native x86 machine check error handling via a notifier
(yes "firmware-first" error handling fails to do the right thing for
the pmem driver), or the ones that are eventually reported via address
range scrub, but only for the nvdimms that implement range scrubbing.
memory_failure() seems a reasonable catch all point to route pfn
failure events, in an arch independent way, to interested drivers.

I'm fine swapping out dax_device blocking_notiier chains for your
proposal, but that does not address all the proposed reworks in my
list which are:

- delete "drivers/acpi/nfit/mce.c"

- teach memory_failure() to be able to communicate range failure

- enable memory_failure() to defer to a filesystem that can say
"critical metadata is impacted, no point in trying to do file-by-file
isolation, bring the whole fs down".

> I have a series I need to finish and send out to do that for block
> devices.  We probably also need the concept of a holder for the dax
> device to make it work nicely, as otherwise we're going to have a bit
> of a mess.

Ok, I'll take a look at adding a holder.

>
> > This obviously does not solve Dave's desire to get this type of error
> > reporting on block_devices, but I think there's nothing stopping a
> > parallel notifier chain from being created for block-devices, but
> > that's orthogonal to requirements and capabilities provided by
> > dax-devices.
>
> FYI, my series could easily accomodate that if we ever get a block
> driver that actually could report such errors.

Sure, whatever we land for a dax_device could easily be adopted for a
block device.