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

Peter Xu
2022-04-14T21:08:24Z

On Thu, Apr 14, 2022 at 01:57:40PM -0700, Andrew Morton wrote:
> On Thu, 14 Apr 2022 12:30:06 -0400 Peter Xu <peterx@redhat.com> wrote:
>
> > > Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
> > >
> > > Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
> >
> > Thanks, Marek, for the fast feedback!
>
> Certainly.
>
> > I've also verified it for the uffd-wp case so the whole series keeps
> > running as usual and nothing else shows up after the new patch replaced.
> >
> > Andrew, any suggestion on how we proceed with the replacement patch?
> > E.g. do you want me to post it separately to the list?
>
> I turned it into an incremental diff and queued it against [03/23]:
>
> --- a/include/linux/mm_types.h~mm-check-against-orig_pte-for-finish_fault-fix
> +++ a/include/linux/mm_types.h
> @@ -814,6 +814,8 @@ typedef struct {
> * @FAULT_FLAG_UNSHARE: The fault is an unsharing request to unshare (and mark
> * exclusive) a possibly shared anonymous page that is
> * mapped R/O.
> + * @FAULT_FLAG_ORIG_PTE_VALID: whether the fault has vmf->orig_pte cached.
> + * We should only access orig_pte if this flag set.
> *
> * About @FAULT_FLAG_ALLOW_RETRY and @FAULT_FLAG_TRIED: we can specify
> * whether we would allow page faults to retry by specifying these two
> @@ -850,6 +852,7 @@ enum fault_flag {
> FAULT_FLAG_INSTRUCTION = 1 << 8,
> FAULT_FLAG_INTERRUPTIBLE = 1 << 9,
> FAULT_FLAG_UNSHARE = 1 << 10,
> + FAULT_FLAG_ORIG_PTE_VALID = 1 << 11,
> };
>
> #endif /* _LINUX_MM_TYPES_H */
> --- a/mm/memory.c~mm-check-against-orig_pte-for-finish_fault-fix
> +++ a/mm/memory.c
> @@ -4194,6 +4194,15 @@ void do_set_pte(struct vm_fault *vmf, st
> set_pte_at(vma->vm_mm, addr, vmf->pte, entry);
> }
>
> +static bool vmf_pte_changed(struct vm_fault *vmf)
> +{
> + if (vmf->flags & FAULT_FLAG_ORIG_PTE_VALID) {
> + return !pte_same(*vmf->pte, vmf->orig_pte);
> + }
> +
> + return !pte_none(*vmf->pte);
> +}
> +
> /**
> * finish_fault - finish page fault once we have prepared the page to fault
> *
> @@ -4252,7 +4261,7 @@ vm_fault_t finish_fault(struct vm_fault
> vmf->address, &vmf->ptl);
> ret = 0;
> /* Re-check under ptl */
> - if (likely(pte_same(*vmf->pte, vmf->orig_pte)))
> + if (likely(!vmf_pte_changed(vmf)))
> do_set_pte(vmf, page, vmf->address);
> else
> ret = VM_FAULT_NOPAGE;
> @@ -4720,13 +4729,7 @@ static vm_fault_t handle_pte_fault(struc
> * 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);
> + vmf->flags &= ~FAULT_FLAG_ORIG_PTE_VALID;
> } else {
> /*
> * If a huge pmd materialized under us just retry later. Use
> @@ -4750,6 +4753,7 @@ static vm_fault_t handle_pte_fault(struc
> */
> vmf->pte = pte_offset_map(vmf->pmd, vmf->address);
> vmf->orig_pte = *vmf->pte;
> + vmf->flags |= FAULT_FLAG_ORIG_PTE_VALID;
>
> /*
> * some architectures can have larger ptes than wordsize,
> _
>

I verified the diff, that matches with what I got. Thanks Andrew.

--
Peter Xu