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Date: Mon, 13 Dec 2021 09:20:20 +0100
From: Christoph Hellwig <hch@lst.de>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Vivek Goyal <vgoyal@redhat.com>, Christoph Hellwig <hch@lst.de>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Dave Jiang <dave.jiang@intel.com>, Alasdair Kergon <agk@redhat.com>,
	Mike Snitzer <snitzer@redhat.com>, Ira Weiny <ira.weiny@intel.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Miklos Szeredi <miklos@szeredi.hu>,
	Matthew Wilcox <willy@infradead.org>,
	device-mapper development <dm-devel@redhat.com>,
	Linux NVDIMM <nvdimm@lists.linux.dev>,
	linux-s390 <linux-s390@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 5/5] dax: always use _copy_mc_to_iter in
 dax_copy_to_iter
Message-ID: <20211213082020.GA21462@lst.de>
References: <20211209063828.18944-1-hch@lst.de> <20211209063828.18944-6-hch@lst.de> <YbNejVRF5NQB0r83@redhat.com> <CAPcyv4i_HdnMcq6MmDMt-a5p=ojh_vsoAiES0vUYEh7HvC1O-A@mail.gmail.com>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Sun, Dec 12, 2021 at 06:48:05AM -0800, Dan Williams wrote:
> On Fri, Dec 10, 2021 at 6:05 AM Vivek Goyal <vgoyal@redhat.com> wrote:
> >
> > On Thu, Dec 09, 2021 at 07:38:28AM +0100, Christoph Hellwig wrote:
> > > While using the MC-safe copy routines is rather pointless on a virtual device
> > > like virtiofs,
> >
> > I was wondering about that. Is it completely pointless.
> >
> > Typically we are just mapping host page cache into qemu address space.
> > That shows as virtiofs device pfn in guest and that pfn is mapped into
> > guest application address space in mmap() call.
> >
> > Given on host its DRAM, so I would not expect machine check on load side
> > so there was no need to use machine check safe variant.
> 
> That's a broken assumption, DRAM experiences multi-bit ECC errors.
> Machine checks, data aborts, etc existed before PMEM.

So the conclusion here is that we should always use the mc safe variant?