linux-kernel - Re: [PATCH 35/46] hugetlb: add MADV

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Message-ID: <Y8nGK2N5E15cenr1@x1n>
Date:   Thu, 19 Jan 2023 17:37:31 -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: [PATCH 35/46] hugetlb: add MADV_COLLAPSE for hugetlb

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