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

[Christoph Hellwig <hch@infradead.org>]

On Tue, Nov 02, 2021 at 12:57:10PM -0700, Dan Williams wrote:
> This goes back to one of the original DAX concerns of wanting a kernel
> library for coordinating PMEM mmap I/O vs leaving userspace to wrap
> PMEM semantics on top of a DAX mapping. The problem is that mmap-I/O
> has this error-handling-API issue whether it is a DAX mapping or not.

Semantics of writes through shared mmaps are a nightmare.  Agreed,
including agreeing that this is neither new nor pmem specific.  But
it also has absolutely nothing to do with the new RWF_ flag.

> CONFIG_ARCH_SUPPORTS_MEMORY_FAILURE implies that processes will
> receive SIGBUS + BUS_MCEERR_A{R,O} when memory failure is signalled
> and then rely on readv(2)/writev(2) to recover. Do you see a readily
> available way to improve upon that model without CPU instruction
> changes? Even with CPU instructions changes, do you think it could
> improve much upon the model of interrupting the process when a load
> instruction aborts?

The "only" think we need is something like the exception table we
use in the kernel for the uaccess helpers (and the new _nofault
kernel access helper).  But I suspect refitting that into userspace
environments is probably non-trivial.

> I do agree with you that DAX needs to separate itself from block, but
> I don't think it follows that DAX also needs to separate itself from
> readv/writev for when a kernel slow-path needs to get involved because
> mmap I/O (just CPU instructions) does not have the proper semantics.
> Even if you got one of the ARCH_SUPPORTS_MEMORY_FAILURE to implement
> those semantics in new / augmented CPU instructions you will likely
> not get all of them to move and certainly not in any near term
> timeframe, so the kernel path will be around indefinitely.

I think you misunderstood me.  I don't think pmem needs to be
decoupled from the read/write path.  But I'm very skeptical of adding
a new flag to the common read/write path for the special workaround
that a plain old write will not actually clear errors unlike every
other store interfac.

> Meanwhile, I think RWF_RECOVER_DATA is generically useful for other
> storage besides PMEM and helps storage-drivers do better than large
> blast radius "I/O error" completions with no other recourse.

How?