linux-kernel - [PATCH v2 10/10] mm/hugetlb: Document why page_vma_mapped

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Date:   Wed,  7 Dec 2022 15:31:58 -0500
From:   Peter Xu <peterx@...hat.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Jann Horn <jannh@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>, peterx@...hat.com,
        James Houghton <jthoughton@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Nadav Amit <nadav.amit@...il.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Muchun Song <songmuchun@...edance.com>
Subject: [PATCH v2 10/10] mm/hugetlb: Document why page_vma_mapped_walk() is safe to walk

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.
+		 */
 		pvmw->pte = hugetlb_walk(vma, pvmw->address, size);
 		if (!pvmw->pte)
 			return false;
-- 
2.37.3