linux-kernel - Re: [PATCH 19/46] hugetlb: add HGM support for follow_hugetlb

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Message-ID: <Y8BLKaKy9CUCy/vy@x1n>
Date:   Thu, 12 Jan 2023 13:02:17 -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 19/46] hugetlb: add HGM support for follow_hugetlb_page

On Thu, Jan 05, 2023 at 10:18:17AM +0000, James Houghton wrote:
> @@ -6731,22 +6746,22 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  		 * and skip the same_page loop below.
>  		 */
>  		if (!pages && !vmas && !pfn_offset &&
> -		    (vaddr + huge_page_size(h) < vma->vm_end) &&
> -		    (remainder >= pages_per_huge_page(h))) {
> -			vaddr += huge_page_size(h);
> -			remainder -= pages_per_huge_page(h);
> -			i += pages_per_huge_page(h);
> +		    (vaddr + pages_per_hpte < vma->vm_end) &&
> +		    (remainder >= pages_per_hpte)) {
> +			vaddr += pages_per_hpte;

This silently breaks hugetlb GUP.. should be

                        vaddr += hugetlb_pte_size(&hpte);

It caused misterious MISSING events when I'm playing with this tree, and
I'm surprised it rooted here.  So far the most time consuming one. :)

> +			remainder -= pages_per_hpte;
> +			i += pages_per_hpte;
>  			spin_unlock(ptl);
>  			hugetlb_vma_unlock_read(vma);
>  			continue;
>  		}

-- 
Peter Xu