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From:   James Houghton <jthoughton@google.com>
Date:   Thu, 12 Jan 2023 13:06:07 -0500
Message-ID: <CADrL8HX6Y5uUDeAViTmU6ysPezGzRtB=P76mxc3RG53_OUaNCw@mail.gmail.com>
Subject: Re: [PATCH 19/46] hugetlb: add HGM support for follow_hugetlb_page
To:     Peter Xu <peterx@redhat.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
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On Thu, Jan 12, 2023 at 1:02 PM Peter Xu <peterx@redhat.com> wrote:
>
> 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. :)

Thanks Peter!! And the `vaddr + pages_per_hpte < vma->vm_end` should
be `vaddr + hugetlb_pte_size(&hpte) < vma->vm_end` too.