Re: [RFC PATCH 00/26] hugetlb: Introduce HugeTLB high-granularity mapping

[Dr. David Alan Gilbert]

* James Houghton (jthoughton@xxxxxxxxxx) wrote:
> On Mon, Jun 27, 2022 at 10:56 AM Dr. David Alan Gilbert
> <dgilbert@xxxxxxxxxx> wrote:
> >
> > * James Houghton (jthoughton@xxxxxxxxxx) wrote:
> > > On Fri, Jun 24, 2022 at 11:29 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > > >
> > > > On Fri, Jun 24, 2022 at 05:36:30PM +0000, James Houghton wrote:
> > > > > [1] This used to be called HugeTLB double mapping, a bad and confusing
> > > > >     name. "High-granularity mapping" is not a great name either. I am open
> > > > >     to better names.
> > > >
> > > > Oh good, I was grinding my teeth every time I read it ;-)
> > > >
> > > > How does "Fine granularity" work for you?
> > > > "sub-page mapping" might work too.
> > >
> > > "Granularity", as I've come to realize, is hard to say, so I think I
> > > prefer sub-page mapping. :) So to recap the suggestions I have so far:
> > >
> > > 1. Sub-page mapping
> > > 2. Granular mapping
> > > 3. Flexible mapping
> > >
> > > I'll pick one of these (or maybe some other one that works better) for
> > > the next version of this series.
> >
> > <shrug> Just a name; SPM might work (although may confuse those
> > architectures which had subprotection for normal pages), and at least
> > we can mispronounce it.
> >
> > In 14/26 your commit message says:
> >
> >   1. Faults can be passed to handle_userfault. (Userspace will want to
> >      use UFFD_FEATURE_REAL_ADDRESS to get the real address to know which
> >      region they should be call UFFDIO_CONTINUE on later.)
> >
> > can you explain what that new UFFD_FEATURE does?
> 
> +cc Nadav Amit <namit@xxxxxxxxxx> to check me here.
> 
> Sorry, this should be UFFD_FEATURE_EXACT_ADDRESS. It isn't a new
> feature, and it actually isn't needed (I will correct the commit
> message). Why it isn't needed is a little bit complicated, though. Let
> me explain:
> 
> Before UFFD_FEATURE_EXACT_ADDRESS was introduced, the address that
> userfaultfd gave userspace for HugeTLB pages was rounded down to be
> hstate-size-aligned. This would have had to change, because userspace,
> to take advantage of HGM, needs to know which 4K piece to install.
> 
> However, after UFFD_FEATURE_EXACT_ADDRESS was introduced[1], the
> address was rounded down to be PAGE_SIZE-aligned instead, even if the
> flag wasn't used. I think this was an unintended change. If the flag
> is used, then the address isn't rounded at all -- that was the
> intended purpose of this flag. Hope that makes sense.

Oh that's 'fun'; right but the need for the less-rounded address makes
sense.

One other thing I thought of; you provide the modified 'CONTINUE'
behaviour, which works for postcopy as long as you use two mappings in
userspace; one protected by userfault, and one which you do the writes
to, and then issue the CONTINUE into the protected mapping; that's fine,
but it's not currently how we have our postcopy code wired up in qemu,
we have one mapping and use UFFDIO_COPY to place the page.
Requiring the two mappings is fine, but it's probably worth pointing out
the need for it somewhere.

Dave

> The new userfaultfd feature, UFFD_FEATURE_MINOR_HUGETLBFS_HGM, informs
> userspace that high-granularity CONTINUEs are available.
> 
> [1] commit 824ddc601adc ("userfaultfd: provide unmasked address on page-fault")
> 
> 
> >
> > Dave
> >
> > --
> > Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
> >
> 
-- 
Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK