On 2024/8/26 23:36, David Hildenbrand wrote:
On 22.08.24 09:13, Qi Zheng wrote:
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 <zhengqi.arch@xxxxxxxxxxxxx>
---
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 <david@xxxxxxxxxx>
Thanks!
