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

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Date:   Tue, 3 Jan 2023 12:06:19 -0500
From:   Peter Xu <peterx@...hat.com>
To:     James Houghton <jthoughton@...gle.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 Tue, Dec 27, 2022 at 12:02:52PM -0500, James Houghton wrote:
> 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.

Sounds good to me, thanks!

-- 
Peter Xu