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From:   James Houghton <jthoughton@google.com>
Date:   Thu, 15 Dec 2022 13:08:21 -0500
Message-ID: <CADrL8HXaw9ci59QvZROEcWsj53QrCFRqsL1=YHrSt8Y2-JE=Kg@mail.gmail.com>
Subject: Re: [RFC PATCH v2 08/47] hugetlb: add HGM enablement functions
To:     Mike Kravetz <mike.kravetz@oracle.com>
Cc:     Muchun Song <songmuchun@bytedance.com>,
        Peter Xu <peterx@redhat.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
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On Thu, Dec 15, 2022 at 12:52 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 12/13/22 10:49, James Houghton wrote:
> > On Mon, Dec 12, 2022 at 7:14 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
> > >
> > > On 10/21/22 16:36, James Houghton wrote:
> > > > Currently it is possible for all shared VMAs to use HGM, but it must be
> > > > enabled first. This is because with HGM, we lose PMD sharing, and page
> > > > table walks require additional synchronization (we need to take the VMA
> > > > lock).
> > >
> > > Not sure yet, but I expect Peter's series will help with locking for
> > > hugetlb specific page table walks.
> >
> > It should make things a little bit cleaner in this series; I'll rebase
> > HGM on top of those patches this week (and hopefully get a v1 out
> > soon).
> >
> > I don't think it's possible to implement MADV_COLLAPSE with RCU alone
> > (as implemented in Peter's series anyway); we still need the VMA lock.
>
> As I continue going through the series, I realize that I am not exactly
> sure what synchronization by the vma lock is required by HGM.  As you are
> aware, it was originally designed to protect against someone doing a
> pmd_unshare and effectively removing part of the page table.  However,
> since pmd sharing is disabled for vmas with HGM enabled (I think?), then
> it might be a good idea to explicitly say somewhere the reason for using
> the lock.

It synchronizes MADV_COLLAPSE for hugetlb (hugetlb_collapse).
MADV_COLLAPSE will take it for writing and free some page table pages,
and high-granularity walks will generally take it for reading. I'll
make this clear in a comment somewhere and in commit messages.

It might be easier if hugetlb_collapse() had the exact same
synchronization as huge_pmd_unshare, where we not only take the VMA
lock for writing, we also take the i_mmap_rw_sem for writing, so
anywhere where hugetlb_walk() is safe, high-granularity walks are also
safe. I think I should just do that for the sake of simplicity.

- James

> --
> Mike Kravetz