Re: [PATCH v2 12/26] userfaultfd: wp: apply _PAGE_UFFD_WP bit

[Jerome Glisse <jglisse@redhat.com>]

On Fri, Feb 22, 2019 at 03:31:35PM +0800, Peter Xu wrote:
> On Thu, Feb 21, 2019 at 12:44:02PM -0500, Jerome Glisse wrote:
> > On Tue, Feb 12, 2019 at 10:56:18AM +0800, Peter Xu wrote:
> > > Firstly, introduce two new flags MM_CP_UFFD_WP[_RESOLVE] for
> > > change_protection() when used with uffd-wp and make sure the two new
> > > flags are exclusively used.  Then,
> > > 
> > >   - For MM_CP_UFFD_WP: apply the _PAGE_UFFD_WP bit and remove _PAGE_RW
> > >     when a range of memory is write protected by uffd
> > > 
> > >   - For MM_CP_UFFD_WP_RESOLVE: remove the _PAGE_UFFD_WP bit and recover
> > >     _PAGE_RW when write protection is resolved from userspace
> > > 
> > > And use this new interface in mwriteprotect_range() to replace the old
> > > MM_CP_DIRTY_ACCT.
> > > 
> > > Do this change for both PTEs and huge PMDs.  Then we can start to
> > > identify which PTE/PMD is write protected by general (e.g., COW or soft
> > > dirty tracking), and which is for userfaultfd-wp.
> > > 
> > > Since we should keep the _PAGE_UFFD_WP when doing pte_modify(), add it
> > > into _PAGE_CHG_MASK as well.  Meanwhile, since we have this new bit, we
> > > can be even more strict when detecting uffd-wp page faults in either
> > > do_wp_page() or wp_huge_pmd().
> > > 
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > 
> > Few comments but still:
> > 
> > Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> 
> Thanks!
> 
> > 
> > > ---
> > >  arch/x86/include/asm/pgtable_types.h |  2 +-
> > >  include/linux/mm.h                   |  5 +++++
> > >  mm/huge_memory.c                     | 14 +++++++++++++-
> > >  mm/memory.c                          |  4 ++--
> > >  mm/mprotect.c                        | 12 ++++++++++++
> > >  mm/userfaultfd.c                     |  8 ++++++--
> > >  6 files changed, 39 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> > > index 8cebcff91e57..dd9c6295d610 100644
> > > --- a/arch/x86/include/asm/pgtable_types.h
> > > +++ b/arch/x86/include/asm/pgtable_types.h
> > > @@ -133,7 +133,7 @@
> > >   */
> > >  #define _PAGE_CHG_MASK	(PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |		\
> > >  			 _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY |	\
> > > -			 _PAGE_SOFT_DIRTY | _PAGE_DEVMAP)
> > > +			 _PAGE_SOFT_DIRTY | _PAGE_DEVMAP | _PAGE_UFFD_WP)
> > >  #define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)
> > 
> > This chunk needs to be in the earlier arch specific patch.
> 
> Indeed.  I'll move it over.
> 
> > 
> > [...]
> > 
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 8d65b0f041f9..817335b443c2 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > 
> > [...]
> > 
> > > @@ -2198,6 +2208,8 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> > >  				entry = pte_mkold(entry);
> > >  			if (soft_dirty)
> > >  				entry = pte_mksoft_dirty(entry);
> > > +			if (uffd_wp)
> > > +				entry = pte_mkuffd_wp(entry);
> > >  		}
> > >  		pte = pte_offset_map(&_pmd, addr);
> > >  		BUG_ON(!pte_none(*pte));
> > 
> > Reading that code and i thought i would be nice if we could define a
> > pte_mask that we can or instead of all those if () entry |= ... but
> > that is just some dumb optimization and does not have any bearing on
> > the present patch. Just wanted to say that outloud.
> 
> (I agree; though I'll just concentrate on the series for now)
> 
> > 
> > 
> > > diff --git a/mm/mprotect.c b/mm/mprotect.c
> > > index a6ba448c8565..9d4433044c21 100644
> > > --- a/mm/mprotect.c
> > > +++ b/mm/mprotect.c
> > > @@ -46,6 +46,8 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> > >  	int target_node = NUMA_NO_NODE;
> > >  	bool dirty_accountable = cp_flags & MM_CP_DIRTY_ACCT;
> > >  	bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
> > > +	bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
> > > +	bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
> > >  
> > >  	/*
> > >  	 * Can be called with only the mmap_sem for reading by
> > > @@ -117,6 +119,14 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> > >  			if (preserve_write)
> > >  				ptent = pte_mk_savedwrite(ptent);
> > >  
> > > +			if (uffd_wp) {
> > > +				ptent = pte_wrprotect(ptent);
> > > +				ptent = pte_mkuffd_wp(ptent);
> > > +			} else if (uffd_wp_resolve) {
> > > +				ptent = pte_mkwrite(ptent);
> > > +				ptent = pte_clear_uffd_wp(ptent);
> > > +			}
> > > +
> > >  			/* Avoid taking write faults for known dirty pages */
> > >  			if (dirty_accountable && pte_dirty(ptent) &&
> > >  					(pte_soft_dirty(ptent) ||
> > > @@ -301,6 +311,8 @@ unsigned long change_protection(struct vm_area_struct *vma, unsigned long start,
> > >  {
> > >  	unsigned long pages;
> > >  
> > > +	BUG_ON((cp_flags & MM_CP_UFFD_WP_ALL) == MM_CP_UFFD_WP_ALL);
> > 
> > Don't you want to abort and return here if both flags are set ?
> 
> Here I would slightly prefer BUG_ON() because current code (any
> userspace syscalls) cannot trigger this without changing the kernel
> (currently the only kernel user of these two flags will be
> mwriteprotect_range but it'll definitely only pass one flag in).  This
> line will be only useful when we add new kernel code (or writting new
> kernel drivers) and it can be used to detect programming errors. In
> that case IMHO BUG_ON() would be more straightforward.
> 

Ok i agree.

Cheers,
Jérôme