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

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Date:   Wed, 16 Nov 2022 15:30:00 -0800
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 Wed, Nov 16, 2022 at 2:28 PM Peter Xu <peterx@...hat.com> wrote:
>
> On Fri, Oct 21, 2022 at 04:36:49PM +0000, James Houghton wrote:
> > Userspace must provide this new feature when it calls UFFDIO_API to
> > enable HGM. Userspace can check if the feature exists in
> > uffdio_api.features, and if it does not exist, the kernel does not
> > support and therefore did not enable HGM.
> >
> > Signed-off-by: James Houghton <jthoughton@...gle.com>
>
> It's still slightly a pity that this can only be enabled by an uffd context
> plus a minor fault, so generic hugetlb users cannot directly leverage this.

The idea here is that, for applications that can conceivably benefit
from HGM, we have a mechanism for enabling it for that application. So
this patch creates that mechanism for userfaultfd/UFFDIO_CONTINUE. I
prefer this approach over something more general like MADV_ENABLE_HGM
or something.

For hwpoison, HGM will be automatically enabled, but that isn't
implemented in this series. We could also extend MADV_DONTNEED to do
high-granularity unmapping in some way, but that also isn't attempted
here. I'm sure that if there are other cases where HGM may be useful,
we can add/change some uapi to make it possible to take advantage HGM.

- James

>
> The patch itself looks good.
>
> --
> Peter Xu
>