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

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Date:   Wed, 21 Dec 2022 14:23:35 -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

James,

On Wed, Nov 16, 2022 at 03:30:00PM -0800, James Houghton wrote:
> 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.

Sorry to get back to this very late - I know this has been discussed since
the very early stage of the feature, but is there any reasoning behind?

When I start to think seriously on applying this to process snapshot with
uffd-wp I found that the minor mode trick won't easily play - normally
that's a case where all the pages were there mapped huge, but when the app
wants UFFDIO_WRITEPROTECT it may want to remap the huge pages into smaller
pages, probably some size that the user can specify.  It'll be non-trivial
to enable HGM during that phase using MINOR mode because in that case the
pages are all mapped.

For the long term, I am just still worried the current interface is still
not as flexible.

-- 
Peter Xu