Re: [PATCH 3/6] userfaultfd: use userfaultfd_*() helpers instead of open coded flag tests

[Mike Rapoport]

On Tue, Aug 25, 2026 at 12:26:39PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Tue, Aug 25, 2026 at 02:19:52PM +0300, Mike Rapoport wrote:
> > > > +/*
> > > > + * Don't do fault around for WP, RWP or MINOR registered uffd range.  For
> > > > + * MINOR registered range, fault around will be a total disaster and ptes can
> > > > + * be installed without notifications; for WP it should mostly be fine as long
> > > > + * as the fault around checks for pte_none() before the installation, however
> > > > + * to be super safe we just forbid it; for RWP, pre-faulted neighbours would
> > > > + * be indistinguishable from accessed pages in PAGEMAP_SCAN (PAGE_IS_ACCESSED)
> > > > + * and pollute the tracked working set, so each page must be populated by its
> > > > + * own fault.
> > > > + */
> > > > +static inline bool uffd_disable_fault_around(struct vm_area_struct *vma)
> > > > +{
> > > > +	return userfaultfd_minor(vma) || userfaultfd_wp(vma) ||
> > > > +	       userfaultfd_rwp(vma);
> > >
> > > This is changing the logic.
> > >
> > > Before we were testing only the flags, now we have:
> > >
> > > static inline bool userfaultfd_rwp(const struct vm_area_struct *vma)
> > > {
> > > 	/*
> > > 	 * Callers gate PAGE_NONE usage on this; PAGE_NONE is a BUILD_BUG()
> > > 	 * without CONFIG_ARCH_HAS_PTE_PROTNONE, so fold to false.
> > > 	 */
> > > 	if (!IS_ENABLED(CONFIG_ARCH_HAS_PTE_PROTNONE))
> > > 		return false;
> > > 	return vma_test_single_mask(vma, VMA_UFFD_RWP);
> > > }
> > >
> > > I.e. adding in a CONFIG_ARCH_HAS_PTE_PROTNONE check.
> >
> > Without CONFIG_ARCH_HAS_PTE_PROTNONE VMA_UFFD_RWP is hardwired to VM_NONE
> > so it's functionally the same ;-)
> 
> Well then you're explicitly removing logic and not mentioning it anywhere
> with a NFC commit.
> 
> So please say so in the commit message.

Ok 

> > > BTW side-note these:
> > >
> > > static inline bool userfaultfd_missing(const struct vm_area_struct *vma)
> > > {
> > > 	return vma_test_any_mask(vma, VMA_UFFD_MISSING);
> > > }
> > >
> > > static inline bool userfaultfd_wp(const struct vm_area_struct *vma)
> > > {
> > > 	return vma_test_any_mask(vma, VMA_UFFD_WP);
> > > }
> > >
> > > static inline bool userfaultfd_minor(const struct vm_area_struct *vma)
> > > {
> > > 	return vma_test_any_mask(vma, VMA_UFFD_MINOR);
> > > }
> > >
> > > Should all use vma_test_single_mask() really :)
> >
> > These are changed anyway in a later patch.
> 
> Yup, but you (potentially) break one of them. Will comment there.
 
Nope, nothing is broken, will comment there.

-- 
Sincerely yours,
Mike.