Re: [PATCH RFC 06/24] userfaultfd: wp: support write protection for userfault vma range

[Jerome Glisse <jglisse@redhat.com>]

On Wed, Jan 23, 2019 at 10:17:45AM +0800, Peter Xu wrote:
> On Tue, Jan 22, 2019 at 12:02:24PM -0500, Jerome Glisse wrote:
> > On Tue, Jan 22, 2019 at 05:39:35PM +0800, Peter Xu wrote:
> > > On Mon, Jan 21, 2019 at 09:05:35AM -0500, Jerome Glisse wrote:
> > > 
> > > [...]
> > > 
> > > > > +	change_protection(dst_vma, start, start + len, newprot,
> > > > > +				!enable_wp, 0);
> > > > 
> > > > So setting dirty_accountable bring us to that code in mprotect.c:
> > > > 
> > > >     if (dirty_accountable && pte_dirty(ptent) &&
> > > >             (pte_soft_dirty(ptent) ||
> > > >              !(vma->vm_flags & VM_SOFTDIRTY))) {
> > > >         ptent = pte_mkwrite(ptent);
> > > >     }
> > > > 
> > > > My understanding is that you want to set write flag when enable_wp
> > > > is false and you want to set the write flag unconditionaly, right ?
> > > 
> > > Right.
> > > 
> > > > 
> > > > If so then you should really move the change_protection() flags
> > > > patch before this patch and add a flag for setting pte write flags.
> > > > 
> > > > Otherwise the above is broken at it will only set the write flag
> > > > for pte that were dirty and i am guessing so far you always were
> > > > lucky because pte were all dirty (change_protection will preserve
> > > > dirtyness) when you write protected them.
> > > > 
> > > > So i believe the above is broken or at very least unclear if what
> > > > you really want is to only set write flag to pte that have the
> > > > dirty flag set.
> > > 
> > > You are right, if we build the tree until this patch it won't work for
> > > all the cases.  It'll only work if the page was at least writable
> > > before and also it's dirty (as you explained).  Sorry to be unclear
> > > about this, maybe I should at least mention that in the commit message
> > > but I totally forgot it.
> > > 
> > > All these problems are solved in later on patches, please feel free to
> > > have a look at:
> > > 
> > >   mm: merge parameters for change_protection()
> > >   userfaultfd: wp: apply _PAGE_UFFD_WP bit
> > >   userfaultfd: wp: handle COW properly for uffd-wp
> > > 
> > > Note that even in the follow up patches IMHO we can't directly change
> > > the write permission since the page can be shared by other processes
> > > (e.g., the zero page or COW pages).  But the general idea is the same
> > > as you explained.
> > > 
> > > I tried to avoid squashing these stuff altogether as explained
> > > previously.  Also, this patch can be seen as a standalone patch to
> > > introduce the new interface which seems to make sense too, and it is
> > > indeed still working in many cases so I see the latter patches as
> > > enhancement of this one.  Please let me know if you still want me to
> > > have all these stuff squashed, or if you'd like me to squash some of
> > > them.
> > 
> > Yeah i have look at those after looking at this one. You should just
> > re-order the patch this one first and then one that add new flag,
> > then ones that add the new userfaultfd feature. Otherwise you are
> > adding a userfaultfd feature that is broken midway ie it is added
> > broken and then you fix it. Some one bisecting thing might get hurt
> > by that. It is better to add and change everything you need and then
> > add the new feature so that the new feature will work as intended.
> > 
> > So no squashing just change the order ie add the userfaultfd code
> > last.
> 
> Yes this makes sense, I'll do that in v2.  Thanks for the suggestion!

Note before doing a v2 i would really like to see some proof of why
you need new page table flag see my reply to:
    userfaultfd: wp: add WP pagetable tracking to x86

As i believe you can identify COW or KSM from UFD write protect with-
out a pte flag.

Cheers,
Jérôme