Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 3 Jan 2023 12:06:19 -0500
From:   Peter Xu <peterx@redhat.com>
To:     James Houghton <jthoughton@google.com>
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        David Hildenbrand <david@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Mina Almasry <almasrymina@google.com>,
        Zach O'Keefe <zokeefe@google.com>,
        Manish Mishra <manish.mishra@nutanix.com>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Yang Shi <shy828301@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v2 33/47] userfaultfd: add
 UFFD_FEATURE_MINOR_HUGETLBFS_HGM
Message-ID: <Y7Rgiypq1m5XBHfz@x1n>
References: <Y6NdN2ADVCcK70ym@x1n>
 <CADrL8HXqE3s4ckxh0OU5onkhystj=1jMTS+S7GFeiO+kwBo0QQ@mail.gmail.com>
 <Y6N9G0Y2j98V8Pnz@monkey>
 <Y6OEQB3dLSa083F6@x1n>
 <Y6OJUtVkvdptEgW7@monkey>
 <CADrL8HX997CguZWkH3vB4+cYjwLc0mYV4GkroE41bCLRqFiPpg@mail.gmail.com>
 <Y6Om/dvlt1Wl2uZw@monkey>
 <CADrL8HU_4ymb4XMBpujyFECwONKpWP0fgZgy7odb+M23QUx6pQ@mail.gmail.com>
 <Y6RqAbGOFEKFUeQr@x1n>
 <CADrL8HWt0s_Ev+QRCVQqdkLbe_jX0X95A=UNYn1Ej8qpr=38ng@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <CADrL8HWt0s_Ev+QRCVQqdkLbe_jX0X95A=UNYn1Ej8qpr=38ng@mail.gmail.com>
Precedence: bulk

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@redhat.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