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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Tue, 5 Apr 2022 18:22:48 -0700
Message-ID: <CAPcyv4gqBmGCQM_u40cR6GVror6NjhxV5Xd7pdHedE2kHwueoQ@mail.gmail.com>
Subject: Re: [PATCH v11 1/8] dax: Introduce holder for dax_device
To:     Jane Chu <jane.chu@oracle.com>
Cc:     "Darrick J. Wong" <djwong@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Shiyang Ruan <ruansy.fnst@fujitsu.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-xfs <linux-xfs@vger.kernel.org>,
        Linux NVDIMM <nvdimm@lists.linux.dev>,
        Linux MM <linux-mm@kvack.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        david <david@fromorbit.com>
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On Tue, Apr 5, 2022 at 5:55 PM Jane Chu <jane.chu@oracle.com> wrote:
>
> On 3/30/2022 9:18 AM, Darrick J. Wong wrote:
> > On Wed, Mar 30, 2022 at 08:49:29AM -0700, Christoph Hellwig wrote:
> >> On Wed, Mar 30, 2022 at 06:58:21PM +0800, Shiyang Ruan wrote:
> >>> As the code I pasted before, pmem driver will subtract its ->data_offset,
> >>> which is byte-based. And the filesystem who implements ->notify_failure()
> >>> will calculate the offset in unit of byte again.
> >>>
> >>> So, leave its function signature byte-based, to avoid repeated conversions.
> >>
> >> I'm actually fine either way, so I'll wait for Dan to comment.
> >
> > FWIW I'd convinced myself that the reason for using byte units is to
> > make it possible to reduce the pmem failure blast radius to subpage
> > units... but then I've also been distracted for months. :/
> >
>
> Yes, thanks Darrick!  I recall that.
> Maybe just add a comment about why byte unit is used?

I think we start with page failure notification and then figure out
how to get finer grained through the dax interface in follow-on
changes. Otherwise, for finer grained error handling support,
memory_failure() would also need to be converted to stop upcasting
cache-line granularity to page granularity failures. The native MCE
notification communicates a 'struct mce' that can be in terms of
sub-page bytes, but the memory management implications are all page
based. I assume the FS implications are all FS-block-size based?