Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness

From: Thomas Gleixner
Date: Sat Nov 07 2015 - 04:25:12 EST

On Sat, 7 Nov 2015, Dan Williams wrote:
> Thanks for that explanation. Peter had alluded to it at KS, but I
> indeed did not know that it was as horrible as milliseconds of
> latency, hmm...

Yes, I was pretty surprised as well. But even if it's just in the
hundreds of microseconds it can be too much for latency sensitive

> One other mitigation that follows on with Dave's plan of per-inode DAX
> control, is to also track when an inode has a writable DAX mmap
> established. With that we could have a REQ_DAX flag to augment
> REQ_FLUSH to potentially reduce committing violence on the cache. In
> an earlier thread I also recall an idea to have an mmap flag that an
> app can use to say "yes, I'm doing a writable DAX mapping, but I'm
> taking care of the cache myself". We could track innocent cpus, but
> I'm thinking that would be a core change to write-protect pages when a
> thread migrates? In general I feel there's a limit for how much
> hardware workaround is reasonable to do in the core kernel vs waiting
> for the platform to offer better options...

One thing vs. the mmaps: We exactly know which CPUs are involved in
that mapping. We know that from the TLB management. So we probably can
make use of that knowledge.

> Sorry if I'm being a bit punchy, but I'm still feeling like I need to
> defend the notion that DAX may just need to be turned off in some
> situations.

That's fine, if there is no reasonable way around it. It just needs to
be documented so people won't be surprised.



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