On Thursday, 16 December 2021 4:50:47 PM AEDT Peter Xu wrote:

On Thu, Dec 16, 2021 at 01:38:33PM +0800, Peter Xu wrote:

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

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

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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.

So one more thing I forgot to mention... Of course above is based on the fact
that orig_pte will be initialized to zero when creating vmf structure, and
that's done in __handle_mm_fault():

	struct vm_fault vmf = {
		.vma = vma,
		.address = address & PAGE_MASK,
		.flags = flags,
		.pgoff = linear_page_index(vma, address),
		.gfp_mask = __get_fault_gfp_mask(vma),
	};

I'm not sure whether I should explicitly set it to pte_val(0), in most C
programs we'll already assume it's a proper reset of orig_pte value in c99
initialization format, but if anyone thinks we should do that explicitly plus
some comments I can do that too.

Ok, that was really my question. Is:

	if (likely(pte_none(*vmf->pte)))

equivalent to:

	if (likely(pte_same(*vmf->pte, __pte(0))))

for every architecture? Looking at Xtensa for example suggests it might not be:

arch/xtensa/include/asm/pgtable.h:
# define pte_none(pte)	 (pte_val(pte) == (_PAGE_CA_INVALID | _PAGE_USER))

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Thanks,

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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;