Re: [PATCH 2/3] mm/cow: optimise pte dirty/accessed bits handling in fork

[Nicholas Piggin]

On Wed, 29 Aug 2018 16:15:37 -0700
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Wed, Aug 29, 2018 at 4:12 PM Nicholas Piggin <npiggin@xxxxxxxxx> wrote:
> >
> > Dirty micro fault seems to be the big one for my Skylake, takes 300
> > nanoseconds per access. Accessed takes about 100. (I think, have to
> > go over my benchmark a bit more carefully and re-test).  
> 
> Yeah, but they only happen for shared areas after fork, which sounds
> like it shouldn't be a big deal in most cases.

You might be right there.

> 
> And I'm not entirely objecting to your patch per se, I just would want
> to keep the accessed bit changes separate from the dirty bit ones.
> 
> *If* somebody has bisectable issues with it (performance or not), it
> will then be clearer what the exact issue is.

Yeah that makes a lot of sense. I'll do a bit more testing and send
Andrew a respin at least with those split (and a good comment for
the dirty bit vs unmap handling that you pointed out).

Thanks,
Nick