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Date:   Thu, 19 Jan 2023 17:37:31 -0500
From:   Peter Xu <peterx@redhat.com>
To:     James Houghton <jthoughton@google.com>
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.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
Subject: Re: [PATCH 35/46] hugetlb: add MADV_COLLAPSE for hugetlb
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References: <20230105101844.1893104-1-jthoughton@google.com>
 <20230105101844.1893104-36-jthoughton@google.com>
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On Thu, Jan 05, 2023 at 10:18:33AM +0000, James Houghton wrote:
> +	/*
> +	 * Grab the VMA lock and mapping sem for writing. This will prevent
> +	 * concurrent high-granularity page table walks, so that we can safely
> +	 * collapse and free page tables.
> +	 *
> +	 * This is the same locking that huge_pmd_unshare requires.
> +	 */
> +	hugetlb_vma_lock_write(vma);
> +	i_mmap_lock_write(vma->vm_file->f_mapping);

One thing I just noticed - do we need the mmap write lock here?  I don't
quickly see what stops another thread from having the mmap read and walking
upon the pgtables being collapsed.

Thanks,

-- 
Peter Xu