linux-kernel - Re: [RFC PATCH v2 08/47] hugetlb: add HGM enablement functions

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Date:   Thu, 15 Dec 2022 09:51:54 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     James Houghton <jthoughton@...gle.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Peter Xu <peterx@...hat.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 08/47] hugetlb: add HGM enablement functions

On 12/13/22 10:49, James Houghton wrote:
> On Mon, Dec 12, 2022 at 7:14 PM Mike Kravetz <mike.kravetz@...cle.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.
-- 
Mike Kravetz