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Date:   Thu, 8 Dec 2022 16:05:09 -0500
From:   Peter Xu <peterx@redhat.com>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Jann Horn <jannh@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        James Houghton <jthoughton@google.com>,
        Rik van Riel <riel@surriel.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Muchun Song <songmuchun@bytedance.com>
Subject: Re: [PATCH v2 10/10] mm/hugetlb: Document why page_vma_mapped_walk()
 is safe to walk
Message-ID: <Y5JRhU+9QqoUuSdR@x1n>
References: <20221207203034.650899-1-peterx@redhat.com>
 <20221207203158.651092-1-peterx@redhat.com>
 <6a970de6-e3da-f57d-14fd-55f65ddcb27d@nvidia.com>
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In-Reply-To: <6a970de6-e3da-f57d-14fd-55f65ddcb27d@nvidia.com>
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        On Wed, Dec 07, 2022 at 04:16:11PM -0800, John Hubbard wrote:
> On 12/7/22 12:31, Peter Xu wrote:
> > Taking vma lock here is not needed for now because all potential hugetlb
> > walkers here should have i_mmap_rwsem held.  Document the fact.
> > 
> > Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >   mm/page_vma_mapped.c | 10 ++++++++--
> >   1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > index e97b2e23bd28..2e59a0419d22 100644
> > --- a/mm/page_vma_mapped.c
> > +++ b/mm/page_vma_mapped.c
> > @@ -168,8 +168,14 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> >   		/* The only possible mapping was handled on last iteration */
> >   		if (pvmw->pte)
> >   			return not_found(pvmw);
> > -
> > -		/* when pud is not present, pte will be NULL */
> > +		/*
> > +		 * NOTE: we don't need explicit lock here to walk the
> > +		 * hugetlb pgtable because either (1) potential callers of
> > +		 * hugetlb pvmw currently holds i_mmap_rwsem, or (2) the
> > +		 * caller will not walk a hugetlb vma (e.g. ksm or uprobe).
> > +		 * When one day this rule breaks, one will get a warning
> > +		 * in hugetlb_walk(), and then we'll figure out what to do.
> > +		 */
> 
> Confused. Is this documentation actually intended to refer to hugetlb_walk()
> itself, or just this call site? If the former, then let's move it over
> to be right before hugetlb_walk().

It is for this specific code path not hugetlb_walk().

The "holds i_mmap_rwsem" here is a true statement (not requirement) because
PVMW rmap walkers always have that.  That satisfies with hugetlb_walk()
requirements already even without holding the vma lock.

-- 
Peter Xu