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

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Message-ID: <Y86lZxGEQOBv3Hmw@x1n>
Date:   Mon, 23 Jan 2023 10:19:03 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.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

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?

-- 
Peter Xu