linux-kernel - Re: [PATCH v3 04/27] mm/userfaultfd: Introduce special pte for unmapped file-backed mem

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Date:   Wed, 9 Jun 2021 10:43:37 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 04/27] mm/userfaultfd: Introduce special pte for
 unmapped file-backed mem

On Wed, Jun 09, 2021 at 11:06:32PM +1000, Alistair Popple wrote:
> On Friday, 28 May 2021 6:19:04 AM AEST Peter Xu wrote:
> 
> [...]
> 
> > diff --git a/include/asm-generic/pgtable_uffd.h b/include/asm-generic/pgtable_uffd.h
> > index 828966d4c281..95e9811ce9d1 100644
> > --- a/include/asm-generic/pgtable_uffd.h
> > +++ b/include/asm-generic/pgtable_uffd.h
> > @@ -2,6 +2,9 @@
> >  #define _ASM_GENERIC_PGTABLE_UFFD_H
> >  
> >  #ifndef CONFIG_HAVE_ARCH_USERFAULTFD_WP
> > +
> > +#define  UFFD_WP_SWP_PTE_SPECIAL  __pte(0)
> > +
> >  static __always_inline int pte_uffd_wp(pte_t pte)
> >  {
> >  	return 0;
> > diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> > index 331d2ccf0bcc..93f932b53a71 100644
> > --- a/include/linux/userfaultfd_k.h
> > +++ b/include/linux/userfaultfd_k.h
> > @@ -145,6 +145,17 @@ extern int userfaultfd_unmap_prep(struct vm_area_struct *vma,
> >  extern void userfaultfd_unmap_complete(struct mm_struct *mm,
> >  				       struct list_head *uf);
> >  
> > +static inline pte_t pte_swp_mkuffd_wp_special(struct vm_area_struct *vma)
> > +{
> > +	WARN_ON_ONCE(vma_is_anonymous(vma));
> > +	return UFFD_WP_SWP_PTE_SPECIAL;
> > +}
> > +
> > +static inline bool pte_swp_uffd_wp_special(pte_t pte)
> > +{
> > +	return pte_same(pte, UFFD_WP_SWP_PTE_SPECIAL);
> > +}
> > +
> 
> Sorry, only just noticed this but do we need to define a different version of
> this helper that returns false for CONFIG_HAVE_ARCH_USERFAULTFD_WP=n to avoid
> spurious matches with __pte(0) on architectures supporting userfaultfd but not
> userfaultfd-wp?

Good point.. Yes we definitely don't want the empty pte to be recognized as the
special pte..  I'll squash below into the same patch:

----8<----
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 489fb375e66c..23ca449240d1 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -177,7 +177,11 @@ static inline pte_t pte_swp_mkuffd_wp_special(struct vm_area_struct *vma)
 
 static inline bool pte_swp_uffd_wp_special(pte_t pte)
 {
+#ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
        return pte_same(pte, UFFD_WP_SWP_PTE_SPECIAL);
+#else
+       return false;
+#fi
 }
 
 #else /* CONFIG_USERFAULTFD */
----8<----

I'll see whether I can give some dry run without HAVE_ARCH_USERFAULTFD_WP but
with USERFAULTFD.

Thanks for spotting that!

-- 
Peter Xu