[PATCH v7 03/23] mm: Check against orig_pte for finish_fault()

Peter Xu
2022-03-04T05:18:04Z

We used to check against none pte in finish_fault(), with the assumption
that the orig_pte is always none pte.

This change prepares us to be able to call do_fault() on !none ptes. For
example, we should allow that to happen for pte marker so that we can restore
information out of the pte markers.

Let's change the "pte_none" check into detecting changes since we fetched
orig_pte. One trivial thing to take care of here is, when pmd==NULL for
the pgtable we may not initialize orig_pte at all in handle_pte_fault().

By default orig_pte will be all zeros however the problem is not all
architectures are using all-zeros for a none pte. pte_clear() will be the
right thing to use here so that we'll always have a valid orig_pte value
for the whole handle_pte_fault() call.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/memory.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 22d24ea7b87d..cdd0d108d3ee 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4135,7 +4135,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
vmf->address, &vmf->ptl);
ret = 0;
/* Re-check under ptl */
- if (likely(pte_none(*vmf->pte)))
+ if (likely(pte_same(*vmf->pte, vmf->orig_pte)))
do_set_pte(vmf, page, vmf->address);
else
ret = VM_FAULT_NOPAGE;
@@ -4600,6 +4600,13 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
* concurrent faults and from rmap lookups.
*/
vmf->pte = NULL;
+ /*
+ * Always initialize orig_pte. This matches with below
+ * code to have orig_pte to be the none pte if pte==NULL.
+ * This makes the rest code to be always safe to reference
+ * it, e.g. in finish_fault() we'll detect pte changes.
+ */
+ pte_clear(vmf->vma->vm_mm, vmf->address, &vmf->orig_pte);
} else {
/*
* If a huge pmd materialized under us just retry later. Use
--
2.32.0