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

Dan Williams
2021-11-02T16:54:41Z

On Tue, Oct 26, 2021 at 11:50 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> On Fri, Oct 22, 2021 at 08:52:55PM +0000, Jane Chu wrote:
> > Thanks - I try to be honest. As far as I can tell, the argument
> > about the flag is a philosophical argument between two views.
> > One view assumes design based on perfect hardware, and media error
> > belongs to the category of brokenness. Another view sees media
> > error as a build-in hardware component and make design to include
> > dealing with such errors.
>
> No, I don't think so. Bit errors do happen in all media, which is
> why devices are built to handle them. It is just the Intel-style
> pmem interface to handle them which is completely broken.

No, any media can report checksum / parity errors. NVME also seems to
do a poor job with multi-bit ECC errors consumed from DRAM. There is
nothing "pmem" or "Intel" specific here.

> > errors in mind from start. I guess I'm trying to articulate why
> > it is acceptable to include the RWF_DATA_RECOVERY flag to the
> > existing RWF_ flags. - this way, pwritev2 remain fast on fast path,
> > and its slow path (w/ error clearing) is faster than other alternative.
> > Other alternative being 1 system call to clear the poison, and
> > another system call to run the fast pwrite for recovery, what
> > happens if something happened in between?
>
> Well, my point is doing recovery from bit errors is by definition not
> the fast path. Which is why I'd rather keep it away from the pmem
> read/write fast path, which also happens to be the (much more important)
> non-pmem read/write path.

I would expect this interface to be useful outside of pmem as a
"failfast" or "try harder to recover" flag for reading over media
errors.