On Thu, Nov 4, 2021 at 10:43 AM Christoph Hellwig [off-list ref] wrote:

On Thu, Nov 04, 2021 at 09:24:15AM -0700, Dan Williams wrote:

quoted

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.

Fair enough.

quoted

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.

It seems the entanglement with 'struct page', error handling, and
reflink made people take notice. Hopefully someone could follow the
same plumbing we're doing for pmem to offer error-rmap help for NVME
badblocks.