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

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