On 2024/8/26 23:36, David Hildenbrand wrote:

On 22.08.24 09:13, Qi Zheng wrote:

quoted

In handle_pte_fault(), we may modify the vmf->pte after acquiring the
vmf->ptl, so convert it to using pte_offset_map_rw_nolock(). But since we
will do the pte_same() check, so there is no need to get pmdval to do
pmd_same() check, just pass a dummy variable to it.

Signed-off-by: Qi Zheng <redacted>
---
  mm/memory.c | 12 ++++++++++--
  1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 93c0c25433d02..7b6071a0e21e2 100644
--- a/mm/memory.c
+++ b/mm/memory.c

@@ -5499,14 +5499,22 @@ static vm_fault_t handle_pte_fault(struct 

vm_fault *vmf)
          vmf->pte = NULL;
          vmf->flags &= ~FAULT_FLAG_ORIG_PTE_VALID;
      } else {
+        pmd_t dummy_pmdval;
+
          /*
           * A regular pmd is established and it can't morph into a huge
           * pmd by anon khugepaged, since that takes mmap_lock in write
           * mode; but shmem or file collapse to THP could still morph
           * it into a huge pmd: just retry later if so.
+         *
+         * Use the maywrite version to indicate that vmf->pte will be
+         * modified, but since we will use pte_same() to detect the
+         * change of the pte entry, there is no need to get pmdval, so
+         * just pass a dummy variable to it.
           */
-        vmf->pte = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd,
-                         vmf->address, &vmf->ptl);
+        vmf->pte = pte_offset_map_rw_nolock(vmf->vma->vm_mm, vmf->pmd,
+                            vmf->address, &dummy_pmdval,
+                            &vmf->ptl);
          if (unlikely(!vmf->pte))
              return 0;
          vmf->orig_pte = ptep_get_lockless(vmf->pte);

No I understand why we don't need the PMD val in these cases ... the PTE 
would also be pte_none() at the point the page table is freed, so we 
would detect the change as well.

Yes.

I do enjoy documenting why we use a dummy value, though. Likely without 
that, new users will just pass NULL and call it a day.

OK, how about the following:

Use the maywrite version to indicate that vmf->pte will be
modified, but since we will use pte_same() to detect the
change of the !pte_none() entry, there is no need to recheck
the pmdval. Here we chooes to pass a dummy variable instead
of NULL, which helps new user think about why this place is
special.

Acked-by: David Hildenbrand <redacted>

Thanks!