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Subject[PATCH v17 13/23] mm: pagewalk: Don't lock PTEs for walk_page_range_novma()
Date
walk_page_range_novma() can be used to walk page tables or the kernel or
for firmware. These page tables may contain entries that are not backed
by a struct page and so it isn't (in general) possible to take the PTE
lock for the pte_entry() callback. So update walk_pte_range() to only
take the lock when no_vma==false by splitting out the inner loop to a
separate function and add a comment explaining the difference to
walk_page_range_novma().

Signed-off-by: Steven Price <steven.price@arm.com>
---
mm/pagewalk.c | 35 ++++++++++++++++++++++++++++-------
1 file changed, 28 insertions(+), 7 deletions(-)

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index d5773465f6da..4b5ee92ba079 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -4,15 +4,12 @@
#include <linux/sched.h>
#include <linux/hugetlb.h>

-static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
- struct mm_walk *walk)
+static int walk_pte_range_inner(pte_t *pte, unsigned long addr,
+ unsigned long end, struct mm_walk *walk)
{
- pte_t *pte;
- int err = 0;
const struct mm_walk_ops *ops = walk->ops;
- spinlock_t *ptl;
+ int err = 0;

- pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
for (;;) {
err = ops->pte_entry(pte, addr, addr + PAGE_SIZE, walk);
if (err)
@@ -22,8 +19,26 @@ static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
break;
pte++;
}
+ return err;
+}
+
+static int walk_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
+ struct mm_walk *walk)
+{
+ pte_t *pte;
+ int err = 0;
+ spinlock_t *ptl;
+
+ if (walk->no_vma) {
+ pte = pte_offset_map(pmd, addr);
+ err = walk_pte_range_inner(pte, addr, end, walk);
+ pte_unmap(pte);
+ } else {
+ pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
+ err = walk_pte_range_inner(pte, addr, end, walk);
+ pte_unmap_unlock(pte, ptl);
+ }

- pte_unmap_unlock(pte, ptl);
return err;
}

@@ -394,6 +409,12 @@ int walk_page_range(struct mm_struct *mm, unsigned long start,
return err;
}

+/*
+ * Similar to walk_page_range() but can walk any page tables even if they are
+ * not backed by VMAs. Because 'unusual' entries may be walked this function
+ * will also not lock the PTEs for the pte_entry() callback. This is useful for
+ * walking the kernel pages tables or page tables for firmware.
+ */
int walk_page_range_novma(struct mm_struct *mm, unsigned long start,
unsigned long end, const struct mm_walk_ops *ops,
void *private)
--
2.20.1
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