linux-kernel - [PATCH RFC v2 08/12] mm/hugetlb: Use hugetlb walker lock in follow_hugetlb

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Date:   Thu, 17 Nov 2022 20:10:21 -0500
From:   Peter Xu <peterx@...hat.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Rik van Riel <riel@...riel.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>, peterx@...hat.com,
        James Houghton <jthoughton@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: [PATCH RFC v2 08/12] mm/hugetlb: Use hugetlb walker lock in follow_hugetlb_page()

Hugetlb walker lock makes sure the pte_t* won't go away from under us.
Some trick is used to release the walker lock slightly earlier when we
found present pte.

Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/hugetlb.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index fc49e3ca8acd..e81af6a46c59 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6280,6 +6280,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			break;
 		}
 
+		hugetlb_walker_lock();
 		/*
 		 * Some archs (sparc64, sh*) have multiple pte_ts to
 		 * each hugepage.  We have to make sure we get the
@@ -6304,6 +6305,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		    !hugetlbfs_pagecache_present(h, vma, vaddr)) {
 			if (pte)
 				spin_unlock(ptl);
+			hugetlb_walker_unlock();
 			remainder = 0;
 			break;
 		}
@@ -6325,6 +6327,8 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
 			if (pte)
 				spin_unlock(ptl);
+			hugetlb_walker_unlock();
+
 			if (flags & FOLL_WRITE)
 				fault_flags |= FAULT_FLAG_WRITE;
 			else if (unshare)
@@ -6367,6 +6371,15 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			continue;
 		}
 
+		/*
+		 * When reach here, it means the pteval is not absent, so
+		 * anyone who wants to free and invalidate the pgtable page
+		 * (aka, pte*) should need to first unmap the entries which
+		 * relies on the pgtable lock.  Since we're holding it,
+		 * we're safe even without the walker lock anymore.
+		 */
+		hugetlb_walker_unlock();
+
 		pfn_offset = (vaddr & ~huge_page_mask(h)) >> PAGE_SHIFT;
 		page = pte_page(huge_ptep_get(pte));
 
-- 
2.37.3