linux-kernel - Re: [RFC PATCH v2 34/47] hugetlb: userfaultfd: add support for high-granularity UFFDIO

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Message-ID: <CADrL8HWfyaQVfcrqrT7Hzt3iHGXiVzNycfCFKpD=YK-YEAX-nA@mail.gmail.com>
Date:   Tue, 27 Dec 2022 11:38:31 -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 34/47] hugetlb: userfaultfd: add support for
 high-granularity UFFDIO_CONTINUE

On Fri, Dec 23, 2022 at 1:38 PM Peter Xu <peterx@...hat.com> wrote:
>
> James,
>
> On Fri, Oct 21, 2022 at 04:36:50PM +0000, James Houghton wrote:
> > +     bool use_hgm = uffd_ctx_has_hgm(&dst_vma->vm_userfaultfd_ctx) &&
> > +             mode == MCOPY_ATOMIC_CONTINUE;
>
> Do you think in your new version use_hgm can work even for MISSING by
> default?

I don't think so -- UFFDIO_COPY will allocate a hugepage, so I'm not
sure if it makes sense to allow it at high-granularity. If UFFDIO_COPY
didn't allocate a new page, then it could make sense (maybe we'd need
a new ioctl or new UFFDIO_COPY mode?). I think it makes most sense to
add this with another series.

Thanks,
- James