linux-kernel - Re: [PATCH 21/46] hugetlb: use struct hugetlb_pte for walk_hugetlb

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Date:   Mon, 23 Jan 2023 09:49:58 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     James Houghton <jthoughton@...gle.com>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.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: [PATCH 21/46] hugetlb: use struct hugetlb_pte for
 walk_hugetlb_range

On 01/23/23 10:19, Peter Xu wrote:
> Hi, Mike,
> 
> On Thu, Jan 19, 2023 at 03:44:25PM -0800, Mike Kravetz wrote:
> > On 01/19/23 18:07, Peter Xu wrote:
> > > 
> > > Actually when revisiting the locks I'm getting a bit confused on whether
> > > the vma lock is needed if pmd sharing is anyway forbidden for HGM.  I
> > > raised a question in the other patch of MADV_COLLAPSE, maybe they're
> > > related questions so we can keep it there.
> > 
> > I can quickly answer that.  Yes.  The vma lock is also being used for
> > fault/truncation synchronization.  Commit e700898fa075 make sure it is
> > even used on architectures that do not support PMD sharing.
> > 
> > I had come up with a rather ugly method of using the fault mutex for
> > fault/truncation synchronization, but using the vma lock was more
> > elegant.
> 
> Thanks for answering, I'll need to read some more on truncation later.
> Before that, since COLLAPSE will already require the i_mmap_rwsem write
> lock already, does it mean it is naturally race-free against truncation
> even without vma lock?

Yes, and thanks for making me take a closer look at COLLAPSE. :)

-- 
Mike Kravetz