linux-kernel - Re: [RFC PATCH v2 35/47] userfaultfd: require UFFD_FEATURE_EXACT

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Message-ID: <CADrL8HV+v9NgJyX87Dd6XMYPqe1mUNqxYxJX520Xr2x9SktdZw@mail.gmail.com>
Date:   Tue, 27 Dec 2022 11:39:50 -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 35/47] userfaultfd: require UFFD_FEATURE_EXACT_ADDRESS
 when using HugeTLB HGM

On Thu, Dec 22, 2022 at 4:47 PM Peter Xu <peterx@...hat.com> wrote:
>
> On Fri, Oct 21, 2022 at 04:36:51PM +0000, James Houghton wrote:
> > @@ -1990,6 +1990,17 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
> >               ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
> >  #ifndef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
> >       uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS_HGM;
> > +#else
> > +
> > +     ret = -EINVAL;
> > +     if ((uffdio_api.features & UFFD_FEATURE_MINOR_HUGETLBFS_HGM) &&
> > +         !(uffdio_api.features & UFFD_FEATURE_EXACT_ADDRESS))
>
> This check needs to be done upon "features" or "ctx_features", rather than
> "uffdio_api.features".  The latter is the one we'll report to the user only.

Ack, thanks Peter. I'm going to drop this patch given the API change
(switching to MADV_SPLIT).

- James