Re: [PATCH v3 10/14] mm: page_vma_mapped_walk: map_pte() use pte_offset_map_rw_nolock()

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On 2024/9/5 20:07, Muchun Song wrote:


On 2024/9/4 16:40, Qi Zheng wrote:
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_rw_nolock(). At
this time, 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.arch@xxxxxxxxxxxxx>
---
  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..f1d73fd448708 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_rw_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);

Forgot to clear pvmw->ptl? Or how about moving the assignment for it
to the place where the pmd_same check is successful?

+        goto again;
+    }
+

Maybe here is the right place to assign pvmw->ptl.

Right, will do in the v4.


Muchun,
Thanks.

      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);





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