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Message-ID: <Y5JRhU+9QqoUuSdR@x1n>
Date: Thu, 8 Dec 2022 16:05:09 -0500
From: Peter Xu <peterx@...hat.com>
To: John Hubbard <jhubbard@...dia.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Jann Horn <jannh@...gle.com>,
Andrea Arcangeli <aarcange@...hat.com>,
James Houghton <jthoughton@...gle.com>,
Rik van Riel <riel@...riel.com>,
Miaohe Lin <linmiaohe@...wei.com>,
Nadav Amit <nadav.amit@...il.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v2 10/10] mm/hugetlb: Document why page_vma_mapped_walk()
is safe to walk
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@...cle.com>
> > Signed-off-by: Peter Xu <peterx@...hat.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
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