Re: [PATCH v1 1/2] mm/userfaultfd: fix uffd-wp handling for THP migration entries

[David Hildenbrand]

On 05.04.23 17:43, Peter Xu wrote:
> On Wed, Apr 05, 2023 at 05:17:31PM +0200, David Hildenbrand wrote:
> > On 05.04.23 17:12, Peter Xu wrote:
> > > On Wed, Apr 05, 2023 at 04:25:34PM +0200, David Hildenbrand wrote:
> > > > Looks like what we fixed for hugetlb in commit 44f86392bdd1 ("mm/hugetlb:
> > > > fix uffd-wp handling for migration entries in hugetlb_change_protection()")
> > > > similarly applies to THP.
> > > >
> > > > Setting/clearing uffd-wp on THP migration entries is not implemented
> > > > properly. Further, while removing migration PMDs considers the uffd-wp
> > > > bit, inserting migration PMDs does not consider the uffd-wp bit.
> > > >
> > > > We have to set/clear independently of the migration entry type in
> > > > change_huge_pmd() and properly copy the uffd-wp bit in
> > > > set_pmd_migration_entry().
> > > >
> > > > Verified using a simple reproducer that triggers migration of a THP, that
> > > > the set_pmd_migration_entry() no longer loses the uffd-wp bit.
> > > >
> > > > Fixes: f45ec5ff16a7 ("userfaultfd: wp: support swap and page migration")
> > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > >
> > > Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
> > >
> > > Thanks, one trivial nitpick:
> > >
> > > > ---
> > > >    mm/huge_memory.c | 14 ++++++++++++--
> > > >    1 file changed, 12 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > > index 032fb0ef9cd1..bdda4f426d58 100644
> > > > --- a/mm/huge_memory.c
> > > > +++ b/mm/huge_memory.c
> > > > @@ -1838,10 +1838,10 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> > > >    	if (is_swap_pmd(*pmd)) {
> > > >    		swp_entry_t entry = pmd_to_swp_entry(*pmd);
> > > >    		struct page *page = pfn_swap_entry_to_page(entry);
> > > > +		pmd_t newpmd;
> > > >    		VM_BUG_ON(!is_pmd_migration_entry(*pmd));
> > > >    		if (is_writable_migration_entry(entry)) {
> > > > -			pmd_t newpmd;
> > > >    			/*
> > > >    			 * A protection check is difficult so
> > > >    			 * just be safe and disable write
> > > > @@ -1855,8 +1855,16 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> > > >    				newpmd = pmd_swp_mksoft_dirty(newpmd);
> > > >    			if (pmd_swp_uffd_wp(*pmd))
> > > >    				newpmd = pmd_swp_mkuffd_wp(newpmd);
> > > > -			set_pmd_at(mm, addr, pmd, newpmd);
> > > > +		} else {
> > > > +			newpmd = *pmd;
> > > >    		}
> > > > +
> > > > +		if (uffd_wp)
> > > > +			newpmd = pmd_swp_mkuffd_wp(newpmd);
> > > > +		else if (uffd_wp_resolve)
> > > > +			newpmd = pmd_swp_clear_uffd_wp(newpmd);
> > > > +		if (!pmd_same(*pmd, newpmd))
> > > > +			set_pmd_at(mm, addr, pmd, newpmd);
> > > >    		goto unlock;
> > > >    	}
> > > >    #endif
> > > > @@ -3251,6 +3259,8 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
> > > >    	pmdswp = swp_entry_to_pmd(entry);
> > > >    	if (pmd_soft_dirty(pmdval))
> > > >    		pmdswp = pmd_swp_mksoft_dirty(pmdswp);
> > > > +	if (pmd_swp_uffd_wp(*pvmw->pmd))
> > > > +		pmdswp = pmd_swp_mkuffd_wp(pmdswp);
> > >
> > > I think it's fine to use *pmd, but maybe still better to use pmdval?  I
> > > worry pmdp_invalidate()) can be something else in the future that may
> > > affect the bit.
> >
> > Wondering how I ended up with that, I realized that it's actually
> > wrong and might have worked by chance for my reproducer on x86.
> >
> > That should make it work:
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index f977c965fdad..fffc953fa6ea 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -3257,7 +3257,7 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
> >          pmdswp = swp_entry_to_pmd(entry);
> >          if (pmd_soft_dirty(pmdval))
> >                  pmdswp = pmd_swp_mksoft_dirty(pmdswp);
> > -       if (pmd_swp_uffd_wp(*pvmw->pmd))
> > +       if (pmd_uffd_wp(pmdval))
> >                  pmdswp = pmd_swp_mkuffd_wp(pmdswp);
> >          set_pmd_at(mm, address, pvmw->pmd, pmdswp);
> >          page_remove_rmap(page, vma, true);
>
> I guess pmd_swp_uffd_wp() just reads the _USER bit 2 which is also set for
> a present pte, but then it sets swp uffd-wp always even if it was not set.

Yes. I modified the reproducer to migrate without uffd-wp first and we 
suddenly gain a uffd-wp bit.

> Yes the change must be squashed in to be correct, with that, my R-b keeps.

Thanks, I will resend later.

--
Thanks,

David / dhildenb