On Thu, Dec 16, 2021 at 04:01:47PM +1100, Alistair Popple wrote:

On Monday, 15 November 2021 6:55:02 PM AEDT Peter Xu wrote:

quoted

We used to check against none pte and in those cases orig_pte should always be
none pte anyway.

Is that always true? From what I can see in handle_pte_fault() orig_pte only
gets initialised in the !pmd_none() case so might not be pte_none.

I believe it's true, otherwise I must have overlooked.

IMHO it's not "when we set orig_pte" that matters - note that finish_fault()
(that this patch modifies) is only called for file-backed memories, and it's
only called in do_fault() where the pte is not mapped at all.

DAX seems to call it too, but still DAX comes from do_fault() too, afaict.

The pte will not be mapped in two cases in handle_pte_fault():

  - When pmd_none

  - When !pmd_none, however if we find that pte_none==true, that's:

        if (pte_none(vmf->orig_pte)) {
                pte_unmap(vmf->pte);
                vmf->pte = NULL;
        }

So when we're already in do_fault(), afaict, orig_pte must be pte_none().
Another side note is that, IIUC pte_none() is a looser check than the
pte_val()==0 and it should be arch dependent.

Thanks,

quoted

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.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index 04662b010005..d5966d9e24c3 100644
--- a/mm/memory.c
+++ b/mm/memory.c

@@ -4052,7 +4052,7 @@ 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;

-- 
Peter Xu