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

[James Houghton]

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.

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
>