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

[James Houghton]

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.

Thanks!
- James