linux-kernel - Re: [RFC PATCH v2 33/47] userfaultfd: add UFFD_FEATURE_MINOR_HUGETLBFS

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Date:   Tue, 27 Dec 2022 12:02:52 -0500
From:   James Houghton <jthoughton@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        "Zach O'Keefe" <zokeefe@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 33/47] userfaultfd: add UFFD_FEATURE_MINOR_HUGETLBFS_HGM

On Thu, Dec 22, 2022 at 9:30 AM Peter Xu <peterx@...hat.com> 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