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

[Christoph Hellwig <hch@infradead.org>]

On Thu, Nov 04, 2021 at 09:24:15AM -0700, Dan Williams wrote:
> No, the big difference with every other modern storage device is
> access to byte-addressable storage. Storage devices get to "cheat"
> with guaranteed minimum 512-byte accesses. So you can arrange for
> writes to always be large enough to scrub the ECC bits along with the
> data. For PMEM and byte-granularity DAX accesses the "sector size" is
> a cacheline and it needed a new CPU instruction before software could
> atomically update data + ECC. Otherwise, with sub-cacheline accesses,
> a RMW cycle can't always be avoided. Such a cycle pulls poison from
> the device on the read and pushes it back out to the media on the
> cacheline writeback.

Indeed.  The fake byte addressability is indeed the problem, and the
fix is to not do that, at least on the second attempt.

> I don't understand what overprovisioning has to do with better error
> management? No other storage device has seen fit to be as transparent
> with communicating the error list and offering ways to proactively
> scrub it. Dave and Darrick rightly saw this and said "hey, the FS
> could do a much better job for the user if it knew about this error
> list". So I don't get what this argument about spare blocks has to do
> with what XFS wants? I.e. an rmap facility to communicate files that
> have been clobbered by cosmic rays and other calamities.

Well, the answer for other interfaces (at least at the gold plated
cost option) is so strong internal CRCs that user visible bits clobbered
by cosmic rays don't realisticly happen.  But it is a problem with the
cheaper ones, and at least SCSI and NVMe offer the error list through
the Get LBA status command (and I bet ATA too, but I haven't looked into
that).  Oddly enough there has never been much interested from the
fs community for those.

> > So far out of the low instrusiveness options Janes' previous series
> > to automatically retry after calling a clear_poison operation seems
> > like the best idea so far.  We just need to also think about what
> > we want to do for direct users of ->direct_access that do not use
> > the mcsafe iov_iter helpers.
> 
> Those exist? Even dm-writecache uses copy_mc_to_kernel().

I'm sorry, I have completely missed that it has been added.  And it's
been in for a whole year..