Re: [PATCH 5/5] dax: always use _copy_mc_to_iter in dax_copy_to_iter

On Mon, Dec 13, 2021 at 12:20 AM Christoph Hellwig [off-list ref] wrote:

On Sun, Dec 12, 2021 at 06:48:05AM -0800, Dan Williams wrote:

quoted

On Fri, Dec 10, 2021 at 6:05 AM Vivek Goyal [off-list ref] wrote:

quoted

On Thu, Dec 09, 2021 at 07:38:28AM +0100, Christoph Hellwig wrote:

quoted

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?

The proposal is one of the following:

1/ any paths not currently using the mc safe variant should continue
not using it to avoid the performance regression on older platforms,
i.e. drop this patch.

2/ add plumbing to switch to mcsafe variant, but only on newer
platforms, incremental new patch

3/ always use the mc safe variant, keep this patch

We could go with 3/ and see who screams, because 3/ is smallest
ongoing maintenance burden. However, I feel like 1/ is the path of
least resistance until the platforms with the need to do 'careful'
copying age out of use.
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