[PATCH v2 08/10] mm/hugetlb: Make walk_hugetlb_range() safe to pmd unshare

Peter Xu
2022-12-07T20:32:26Z

Since walk_hugetlb_range() walks the pgtable, it needs the vma lock
to make sure the pgtable page will not be freed concurrently.

Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
arch/s390/mm/gmap.c | 2 ++
fs/proc/task_mmu.c | 2 ++
include/linux/pagewalk.h | 11 ++++++++++-
mm/hmm.c | 15 ++++++++++++++-
mm/pagewalk.c | 2 ++
5 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
index 8947451ae021..292a54c490d4 100644
--- a/arch/s390/mm/gmap.c
+++ b/arch/s390/mm/gmap.c
@@ -2643,7 +2643,9 @@ static int __s390_enable_skey_hugetlb(pte_t *pte, unsigned long addr,
end = start + HPAGE_SIZE - 1;
__storage_key_init_range(start, end);
set_bit(PG_arch_1, &page->flags);
+ hugetlb_vma_unlock_read(walk->vma);
cond_resched();
+ hugetlb_vma_lock_read(walk->vma);
return 0;
}

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index e35a0398db63..cf3887fb2905 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1613,7 +1613,9 @@ static int pagemap_hugetlb_range(pte_t *ptep, unsigned long hmask,
frame++;
}

+ hugetlb_vma_unlock_read(walk->vma);
cond_resched();
+ hugetlb_vma_lock_read(walk->vma);

return err;
}
diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
index 959f52e5867d..27a6df448ee5 100644
--- a/include/linux/pagewalk.h
+++ b/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
diff --git a/mm/hmm.c b/mm/hmm.c
index 3850fb625dda..796de6866089 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -493,8 +493,21 @@ static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask,
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);
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index 7f1c9b274906..d98564a7be57 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -302,6 +302,7 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
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 long addr, unsigned long end,
if (err)
break;
} while (addr = next, addr != end);
+ hugetlb_vma_unlock_read(vma);

return err;
}
--
2.37.3