Re: [RFC PATCH v2 33/47] userfaultfd: add UFFD_FEATURE_MINOR_HUGETLBFS_HGM

[Peter Xu]

On Tue, Dec 27, 2022 at 12:02:52PM -0500, James Houghton wrote:
> On Thu, Dec 22, 2022 at 9:30 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
> >
> > On Wed, Dec 21, 2022 at 08:24:45PM -0500, James Houghton wrote:
> > > Not sure what the best name for this flag is either. MADV_ENABLE_HGM
> > > sounds ok. MADV_HUGETLB_HGM or MADV_HUGETLB_SMALL_PAGES could work
> > > too. No need to figure it out now.
> >
> > One more option to consider is MADV_SPLIT (hopefully to be more generic).
> >
> > We already decided to reuse thp MADV_COLLAPSE, we can also introduce
> > MADV_SPLIT and leave thp for later if it can be anything helpful (I
> > remember we used to discuss this for thp split).
> >
> > For hugetlb one SPLIT should enable hgm advise bit on the vma forever.
> 
> MADV_SPLIT sounds okay to me -- we'll see how it turns out when I send
> v1. However, there's an interesting API question regarding what
> address userfaultfd provides. We previously required
> UFFD_FEATURE_EXACT_ADDRESS when you specified
> UFFD_FEATURE_MINOR_HUGETLBFS_HGM so that there was no ambiguity. Now,
> we can do:
> 
> 1. When MADV_SPLIT is given, userfaultfd will now round addresses to
> PAGE_SIZE instead of huge_page_size(hstate), and
> UFFD_FEATURE_EXACT_ADDRESS is not needed.
> 2. Don't change anything. A user must know to provide
> UFFD_FEATURE_EXACT_ADDRESS to get the real address, otherwise they get
> an (unusable) hugepage-aligned address.
> 
> I think #1 sounds fine; let me know if you disagree.

Sounds good to me, thanks!

-- 
Peter Xu