[PATCH 10/14] mm: page_vma_mapped_walk: map_pte() use pte_offset_map_maywrite_nolock()

[Qi Zheng]

In the caller of map_pte(), we may modify the pvmw->pte after acquiring
the pvmw->ptl, so convert it to using pte_offset_map_maywrite_nolock(). At
this time, the write lock of mmap_lock is not held, and the pte_same()
check is not performed after the pvmw->ptl held, so we should get pmdval
and do pmd_same() check to ensure the stability of pvmw->pmd.

Signed-off-by: Qi Zheng <zhengqi.a...@bytedance.com>
---
 mm/page_vma_mapped.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index ae5cc42aa2087..da806f3a5047d 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -13,9 +13,11 @@ static inline bool not_found(struct page_vma_mapped_walk 
*pvmw)
        return false;
 }
 
-static bool map_pte(struct page_vma_mapped_walk *pvmw, spinlock_t **ptlp)
+static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
+                   spinlock_t **ptlp)
 {
        pte_t ptent;
+       pmd_t pmdval;
 
        if (pvmw->flags & PVMW_SYNC) {
                /* Use the stricter lookup */
@@ -25,6 +27,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, 
spinlock_t **ptlp)
                return !!pvmw->pte;
        }
 
+again:
        /*
         * It is important to return the ptl corresponding to pte,
         * in case *pvmw->pmd changes underneath us; so we need to
@@ -32,10 +35,11 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, 
spinlock_t **ptlp)
         * proceeds to loop over next ptes, and finds a match later.
         * Though, in most cases, page lock already protects this.
         */
-       pvmw->pte = pte_offset_map_nolock(pvmw->vma->vm_mm, pvmw->pmd,
-                                         pvmw->address, ptlp);
+       pvmw->pte = pte_offset_map_maywrite_nolock(pvmw->vma->vm_mm, pvmw->pmd,
+                                                  pvmw->address, &pmdval, 
ptlp);
        if (!pvmw->pte)
                return false;
+       *pmdvalp = pmdval;
 
        ptent = ptep_get(pvmw->pte);
 
@@ -69,6 +73,12 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, 
spinlock_t **ptlp)
        }
        pvmw->ptl = *ptlp;
        spin_lock(pvmw->ptl);
+
+       if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pvmw->pmd)))) {
+               spin_unlock(pvmw->ptl);
+               goto again;
+       }
+
        return true;
 }
 
@@ -278,7 +288,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
                        step_forward(pvmw, PMD_SIZE);
                        continue;
                }
-               if (!map_pte(pvmw, &ptl)) {
+               if (!map_pte(pvmw, &pmde, &ptl)) {
                        if (!pvmw->pte)
                                goto restart;
                        goto next_pte;
@@ -307,6 +317,12 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk 
*pvmw)
                if (!pvmw->ptl) {
                        pvmw->ptl = ptl;
                        spin_lock(pvmw->ptl);
+                       if (unlikely(!pmd_same(pmde, 
pmdp_get_lockless(pvmw->pmd)))) {
+                               pte_unmap_unlock(pvmw->pte, pvmw->ptl);
+                               pvmw->ptl = NULL;
+                               pvmw->pte = NULL;
+                               goto restart;
+                       }
                }
                goto this_pte;
        } while (pvmw->address < end);
-- 
2.20.1