Re: Deep flush support for CXL pmem?

[<dan.j.williams@intel.com>]

Huang, Ying wrote:
> Hi, All,
> 
> When reading the documentation of PMDK (https://github.com/pmem/pmdk), I
> found that deep flush is used for data loss recovery in addition to GPF.
> With that, we can identify that a file isn't affected by the GPF failure
> because it wasn't opened during the GPF failure.
> 
> However, IIUC, the Linux kernel doesn't support deep flush (via
> nvdimm_flush()) for CXL pmem at least for now.  For NVDIMM, we have WPQ,
> but it appears that we don't have that for CXL.  Is deep flush for CXL
> defined in some spec?

No, and I think a dynamic "deep flush" mechanism is a mistake that CXL
should not pursue. (personal opinion, not necessarily opinion of
$employer)

"All storage is a lie" applies here in that all a deep flush mechanism
does is maybe reduce the occurrence of dirty-shutdown failures. It can
not guarantee that some other event causes the write to be dropped.

CXL does have a Global Persistence Flush mechanism that fires as the
system is dying, but it is not something that can be triggered at run
time.

So, either the system has the reserve energy to make sure that all
globally visible writes make it to persistent storage or it does not. If
it does not then it had better arrange for those failures to be flagged
via a dirty shutdown mechanism.

If a platform fires dirty-shutdown events at a significantly higher rate
than say the capacitor-protected write-cache on an SSD, then that feels
like an "improve the hardware" problem, not a "teach the software to
chase writes with a deep flush and hope it helps" problem.

See discussions like this for the last time mapping pmem deep flush was
attempted to be mapped to storage semantics like FUA (Force Unit
Access):

http://lore.kernel.org/YtefnyIvY9OdrVU5@infradead.org