The patch titled
Subject: mm/hugetlb: make walk_hugetlb_range() safe to pmd unshare
has been added to the -mm mm-unstable branch. Its filename is
mm-hugetlb-make-walk_hugetlb_range-safe-to-pmd-unshare.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb-make-walk_hugetlb_range-safe-to-pmd-unshare.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Peter Xu <peterx@xxxxxxxxxx>
Subject: mm/hugetlb: make walk_hugetlb_range() safe to pmd unshare
Date: Fri, 16 Dec 2022 10:52:26 -0500
Since walk_hugetlb_range() walks the pgtable, it needs the vma lock to
make sure the pgtable page will not be freed concurrently.
Link: https://lkml.kernel.org/r/20221216155226.2043738-1-peterx@xxxxxxxxxx
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: James Houghton <jthoughton@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/pagewalk.h | 11 ++++++++++-
mm/hmm.c | 15 ++++++++++++++-
mm/pagewalk.c | 2 ++
3 files changed, 26 insertions(+), 2 deletions(-)
--- a/include/linux/pagewalk.h~mm-hugetlb-make-walk_hugetlb_range-safe-to-pmd-unshare
+++ a/include/linux/pagewalk.h
@@ -21,7 +21,16 @@ struct mm_walk;
* depth is -1 if not known, 0:PGD, 1:P4D, 2:PUD, 3:PMD.
* Any folded depths (where PTRS_PER_P?D is equal to 1)
* are skipped.
- * @hugetlb_entry: if set, called for each hugetlb entry
+ * @hugetlb_entry: if set, called for each hugetlb entry. This hook
+ * function is called with the vma lock held, in order to
+ * protect against a concurrent freeing of the pte_t* or
+ * the ptl. In some cases, the hook function needs to drop
+ * and retake the vma lock in order to avoid deadlocks
+ * while calling other functions. In such cases the hook
+ * function must either refrain from accessing the pte or
+ * ptl after dropping the vma lock, or else revalidate
+ * those items after re-acquiring the vma lock and before
+ * accessing them.
* @test_walk: caller specific callback function to determine whether
* we walk over the current vma or not. Returning 0 means
* "do page table walk over the current vma", returning
--- a/mm/hmm.c~mm-hugetlb-make-walk_hugetlb_range-safe-to-pmd-unshare
+++ a/mm/hmm.c
@@ -493,8 +493,21 @@ static int hmm_vma_walk_hugetlb_entry(pt
required_fault =
hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, cpu_flags);
if (required_fault) {
+ int ret;
+
spin_unlock(ptl);
- return hmm_vma_fault(addr, end, required_fault, walk);
+ hugetlb_vma_unlock_read(vma);
+ /*
+ * Avoid deadlock: drop the vma lock before calling
+ * hmm_vma_fault(), which will itself potentially take and
+ * drop the vma lock. This is also correct from a
+ * protection point of view, because there is no further
+ * use here of either pte or ptl after dropping the vma
+ * lock.
+ */
+ ret = hmm_vma_fault(addr, end, required_fault, walk);
+ hugetlb_vma_lock_read(vma);
+ return ret;
}
pfn = pte_pfn(entry) + ((start & ~hmask) >> PAGE_SHIFT);
--- a/mm/pagewalk.c~mm-hugetlb-make-walk_hugetlb_range-safe-to-pmd-unshare
+++ a/mm/pagewalk.c
@@ -302,6 +302,7 @@ static int walk_hugetlb_range(unsigned l
const struct mm_walk_ops *ops = walk->ops;
int err = 0;
+ hugetlb_vma_lock_read(vma);
do {
next = hugetlb_entry_end(h, addr, end);
pte = huge_pte_offset(walk->mm, addr & hmask, sz);
@@ -314,6 +315,7 @@ static int walk_hugetlb_range(unsigned l
if (err)
break;
} while (addr = next, addr != end);
+ hugetlb_vma_unlock_read(vma);
return err;
}
_
Patches currently in -mm which might be from peterx@xxxxxxxxxx are
mm-uffd-fix-pte-marker-when-fork-without-fork-event.patch
mm-fix-a-few-rare-cases-of-using-swapin-error-pte-marker.patch
mm-uffd-always-wr-protect-pte-in-ptepmd_mkuffd_wp.patch
mm-hugetlb-let-vma_offset_start-to-return-start.patch
mm-hugetlb-dont-wait-for-migration-entry-during-follow-page.patch
mm-hugetlb-document-huge_pte_offset-usage.patch
mm-hugetlb-move-swap-entry-handling-into-vma-lock-when-faulted.patch
mm-hugetlb-make-userfaultfd_huge_must_wait-safe-to-pmd-unshare.patch
mm-hugetlb-make-hugetlb_follow_page_mask-safe-to-pmd-unshare.patch
mm-hugetlb-make-follow_hugetlb_page-safe-to-pmd-unshare.patch
mm-hugetlb-make-walk_hugetlb_range-safe-to-pmd-unshare.patch
mm-hugetlb-introduce-hugetlb_walk.patch
