Re: [dm-devel] [PATCH 0/6] dax poison recovery with RWF_RECOVERY_DATA flag

[Christoph Hellwig <hch@infradead.org>]

On Wed, Nov 03, 2021 at 11:21:39PM -0700, Dan Williams wrote:
> The concern I have with dax_clear_poison() is that it precludes atomic
> error clearing.

atomic as in clear poison and write the actual data?  Yes, that would
be useful, but it is not how the Intel pmem support actually works, right?

> Also, as Boris and I discussed, poisoned pages should
> be marked NP (not present) rather than UC (uncacheable) [1].

This would not really have an affect on the series, right?  But yes,
that seems like the right thing to do.

> With
> those 2 properties combined I think that wants a custom pmem fault
> handler that knows how to carefully write to pmem pages with poison
> present, rather than an additional explicit dax-operation. That also
> meets Christoph's requirement of "works with the intended direct
> memory map use case".

So we have 3 kinds of accesses to DAX memory:

 (1) user space mmap direct access. 
 (2) iov_iter based access (could be from kernel or userspace)
 (3) open coded kernel access using ->direct_access

One thing I noticed:  (2) could also work with kernel memory or pages,
but that doesn't use MC safe access.  Which seems like a major independent
of this discussion.

I suspect all kernel access could work fine with a copy_mc_to_kernel
helper as long as everyone actually uses it, which will cover the
missing required bits of (2) and (3) together with something like the
->clear_poison series from Jane. We just need to think hard what we
want to do for userspace mmap access.