Re: [RFC PATCH 01/14] userfaultfd: set dirty and young on writeprotect

[Peter Xu]

On Wed, Jul 20, 2022 at 11:39:23AM +0200, David Hildenbrand wrote:
> On 19.07.22 22:47, Peter Xu wrote:
> > On Mon, Jul 18, 2022 at 05:01:59AM -0700, Nadav Amit wrote:
> >> From: Nadav Amit <namit@xxxxxxxxxx>
> >>
> >> When userfaultfd makes a PTE writable, it can now change the PTE
> >> directly, in some cases, without going triggering a page-fault first.
> >> Yet, doing so might leave the PTE that was write-unprotected as old and
> >> clean. At least on x86, this would cause a >500 cycles overhead when the
> >> PTE is first accessed.
> >>
> >> Use MM_CP_WILL_NEED to set the PTE as young and dirty when userfaultfd
> >> gets a hint that the page is likely to be used. Avoid changing the PTE
> >> to young and dirty in other cases to avoid excessive writeback and
> >> messing with the page reclamation logic.
> >>
> >> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> >> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> >> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> >> Cc: David Hildenbrand <david@xxxxxxxxxx>
> >> Cc: Peter Xu <peterx@xxxxxxxxxx>
> >> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >> Cc: Will Deacon <will@xxxxxxxxxx>
> >> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> >> Cc: Nick Piggin <npiggin@xxxxxxxxx>
> >> ---
> >>  include/linux/mm.h | 2 ++
> >>  mm/mprotect.c      | 9 ++++++++-
> >>  mm/userfaultfd.c   | 8 ++++++--
> >>  3 files changed, 16 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >> index 9cc02a7e503b..4afd75ce5875 100644
> >> --- a/include/linux/mm.h
> >> +++ b/include/linux/mm.h
> >> @@ -1988,6 +1988,8 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
> >>  /* Whether this change is for write protecting */
> >>  #define  MM_CP_UFFD_WP                     (1UL << 2) /* do wp */
> >>  #define  MM_CP_UFFD_WP_RESOLVE             (1UL << 3) /* Resolve wp */
> >> +/* Whether to try to mark entries as dirty as they are to be written */
> >> +#define  MM_CP_WILL_NEED		   (1UL << 4)
> >>  #define  MM_CP_UFFD_WP_ALL                 (MM_CP_UFFD_WP | \
> >>  					    MM_CP_UFFD_WP_RESOLVE)
> >>  
> >> diff --git a/mm/mprotect.c b/mm/mprotect.c
> >> index 996a97e213ad..34c2dfb68c42 100644
> >> --- a/mm/mprotect.c
> >> +++ b/mm/mprotect.c
> >> @@ -82,6 +82,7 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
> >>  	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;
> >> +	bool will_need = cp_flags & MM_CP_WILL_NEED;
> >>  
> >>  	tlb_change_page_size(tlb, PAGE_SIZE);
> >>  
> >> @@ -172,6 +173,9 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
> >>  				ptent = pte_clear_uffd_wp(ptent);
> >>  			}
> >>  
> >> +			if (will_need)
> >> +				ptent = pte_mkyoung(ptent);
> > 
> > For uffd path, UFFD_FLAGS_ACCESS_LIKELY|UFFD_FLAGS_WRITE_LIKELY are new
> > internal flags used with or without the new feature bit set.  It means even
> > with !ACCESS_HINT we'll start to set young bit while we used not to?  Is
> > that some kind of a light abi change?
> > 
> > I'd suggest we only set will_need if ACCESS_HINT is set.
> > 
> >> +
> >>  			/*
> >>  			 * In some writable, shared mappings, we might want
> >>  			 * to catch actual write access -- see
> >> @@ -187,8 +191,11 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
> >>  			 */
> >>  			if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
> >>  			    !pte_write(ptent) &&
> >> -			    can_change_pte_writable(vma, addr, ptent))
> >> +			    can_change_pte_writable(vma, addr, ptent)) {
> >>  				ptent = pte_mkwrite(ptent);
> >> +				if (will_need)
> >> +					ptent = pte_mkdirty(ptent);
> > 
> > Can we make this unconditional?  IOW to cover both:
> > 
> >   (1) When will_need is not set, or
> >   (2) mprotect() too
> > 
> > David's patch is good in that we merged the unprotect and CoW.  However
> > that's not complete because the dirty bit ops are missing.
> > 
> > Here IMHO we should have a standalone patch to just add the dirty bit into
> > this logic when we'll grant write bit.  IMHO it'll make the write+dirty
> > bits coherent again in all paths.
> 
> I'm not sure I follow.
> 
> We *surely* don't want to dirty random pages (especially once in the
> pagecache/swapcache) simply because we change protection.
> 
> Just like we don't set all pages write+dirty in a writable VMA on a read
> fault.

IMO unmprotect (in generic mprotect form or uffd form) has a stronger sign
of page being written, unlike read faults, as many of them happen because
page being written and message generated.

But yeah you have a point too, maybe we shouldn't assume such a condition.
Especially as long as we won't set write=1 without soft-dirty tracking
enabled, I think it should be safe.

-- 
Peter Xu