linux-kernel - Re: [RFC PATCH v2 06/47] hugetlb: extend vma lock for shared vmas

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Date:   Wed, 30 Nov 2022 16:01:04 -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 06/47] hugetlb: extend vma lock for shared vmas

On Fri, Oct 21, 2022 at 04:36:22PM +0000, James Houghton wrote:
> This allows us to add more data into the shared structure, which we will
> use to store whether or not HGM is enabled for this VMA or not, as HGM
> is only available for shared mappings.
> 
> It may be better to include HGM as a VMA flag instead of extending the
> VMA lock structure.
> 
> Signed-off-by: James Houghton <jthoughton@...gle.com>
> ---
>  include/linux/hugetlb.h |  4 +++
>  mm/hugetlb.c            | 65 +++++++++++++++++++++--------------------
>  2 files changed, 37 insertions(+), 32 deletions(-)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index a899bc76d677..534958499ac4 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -121,6 +121,10 @@ struct hugetlb_vma_lock {
>  	struct vm_area_struct *vma;
>  };
>  
> +struct hugetlb_shared_vma_data {
> +	struct hugetlb_vma_lock vma_lock;
> +};

How about add a comment above hugetlb_vma_lock showing how it should be
used correctly?  We lacked documents on the lock for pmd sharing
protections, now if to reuse the same lock for HGM pgtables I think some
doc will definitely help.

To summarize, I think so far it means:

  - Read lock needed when one wants to stablize VM_SHARED pgtables (covers
    both pmd shared pgtables or hgm low-level pgtables)

  - Write lock needed when one wants to release VM_SHARED pgtable pages
    (covers both pmd unshare or releasing hgm low-level pgtables)

Or something like that.

-- 
Peter Xu