Re: [PATCH v4] mm: improve mprotect(R|W) efficiency on pages referenced once

[Peter Xu <peterx@redhat.com>]

On Thu, May 27, 2021 at 04:37:11PM -0700, Peter Collingbourne wrote:
> On Thu, May 27, 2021 at 2:41 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > Peter,
> >
> > On Thu, May 27, 2021 at 12:04:53PM -0700, Peter Collingbourne wrote:
> >
> > [...]
> >
> > > +static bool may_avoid_write_fault(pte_t pte, struct vm_area_struct *vma,
> > > +                               unsigned long cp_flags)
> > > +{
> > > +     if (!(cp_flags & MM_CP_DIRTY_ACCT)) {
> > > +             if (!(vma_is_anonymous(vma) && (vma->vm_flags & VM_WRITE)))
> > > +                     return false;
> > > +
> > > +             if (page_count(pte_page(pte)) != 1)
> > > +                     return false;
> > > +     }
> >
> > Can we make MM_CP_DIRTY_ACCT still in charge?  IIUC that won't affect your use
> > case, something like:
> >
> >        /* Never apply trick if MM_CP_DIRTY_ACCT not set */
> >        if (!(cp_flags & MM_CP_DIRTY_ACCT))
> >            return false;
> >
> > The thing is that's really what MM_CP_DIRTY_ACCT is about, imho (as its name
> > shows).  Say, we should start to count on the dirty bit for applying the write
> > bit only if that flag set.  With above, I think we can drop the pte_uffd_wp()
> > check below because uffd_wp never applies MM_CP_DIRTY_ACCT when do
> > change_protection().
> 
> I don't think that would work. The anonymous pages that we're
> interesting in optimizing are private writable pages, for which
> vma_wants_writenotify(vma, vma->vm_page_prot) would return false (and
> thus MM_CP_DIRTY_ACCT would not be set, and thus your code would
> disable the optimization), because of this code at the top of
> vma_wants_writenotify:
> 
>         /* If it was private or non-writable, the write bit is already clear */
>         if ((vm_flags & (VM_WRITE|VM_SHARED)) != ((VM_WRITE|VM_SHARED)))
>                 return 0;
> 
> IIUC, dirty accountable is about whether we can always apply the
> optimization no matter what the ref count is, so it isn't suitable for
> situations where we need to check the ref count.

Ah I see.. Though it still looks weird e.g. the first check could have been
done before calling change_protection()?

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 96f4df023439..9270140afbbd 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -548,7 +548,8 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
         * held in write mode.
         */
        vma->vm_flags = newflags;
-       dirty_accountable = vma_wants_writenotify(vma, vma->vm_page_prot);
+       dirty_accountable = vma_wants_writenotify(vma, vma->vm_page_prot) ||
+           (vma_is_anonymous(vma) && (vma->vm_flags & VM_WRITE));
        vma_set_page_prot(vma);
 
        change_protection(vma, start, end, vma->vm_page_prot,

Would something like this make the check even faster?

Meanwhile when I'm looking at the rest I found I cannot understand the other
check in this patch regarding soft dirty:

+       if (!pte_soft_dirty(pte) && (vma->vm_flags & VM_SOFTDIRTY))
+               return false;

I'm wondering why it's not:

+       if (!pte_soft_dirty(pte) && !(vma->vm_flags & VM_SOFTDIRTY))
+               return false;

Then I look back and it's indeed what it does before, starting from commit
64e455079e1b ("mm: softdirty: enable write notifications on VMAs after
VM_SOFTDIRTY cleared", 2014-10-14):

        if (dirty_accountable && pte_dirty(ptent) &&
                (pte_soft_dirty(ptent) ||
                !(vma->vm_flags & VM_SOFTDIRTY)))

However I don't get it... Shouldn't this be "if soft dirty set, or soft dirty
tracking not enabled, then we can grant the write bit"?  The thing is afaiu
VM_SOFTDIRTY works in the reversed way that soft dirty enabled only if it's
cleared.  Hmm... Am I the only one thinks it wrong?

-- 
Peter Xu