linux-kernel - Re: [dm-devel] [PATCH 0/6] dax poison recovery with RWF_RECOVERY

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Date:   Thu, 4 Nov 2021 01:36:47 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jane Chu <jane.chu@...cle.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        "david@...morbit.com" <david@...morbit.com>,
        "vishal.l.verma@...el.com" <vishal.l.verma@...el.com>,
        "dave.jiang@...el.com" <dave.jiang@...el.com>,
        "agk@...hat.com" <agk@...hat.com>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        "ira.weiny@...el.com" <ira.weiny@...el.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "vgoyal@...hat.com" <vgoyal@...hat.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>
Subject: Re: [dm-devel] [PATCH 0/6] dax poison recovery with
 RWF_RECOVERY_DATA flag

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.