Re: [PATCH v8 03/23] mm: Check against orig_pte for finish_fault()

Alistair Popple
2022-04-12T02:06:45Z

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Looks ok to me now and should work for all architectures.

Reviewed-by: Alistair Popple <apopple@nvidia.com>

Peter Xu <peterx@redhat.com> writes:

> 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 3f396241a7db..b1af996b09ca 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4241,7 +4241,7 @@ 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;
> @@ -4709,6 +4709,13 @@ 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