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

[Peter Xu <peterx@redhat.com>]

On Thu, May 27, 2021 at 06:35:42PM -0700, Peter Collingbourne wrote:
> On Thu, May 27, 2021 at 6:21 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > 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?
> 
> That still doesn't seem like it would work either. I think we need
> three kinds of behavior (glossing over a bunch of details):
> 
> - always make RW for certain shared pages (this is the original dirty
> accountable behavior)
> - don't make RW except for page_count==1 for certain private pages
> - don't optimize at all in other cases
> 
> A single bit isn't enough to cover all of these possibilities.

Yes I guess you're right.

> 
> > 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?
> 
> No strong opinions here, I'm just preserving the original logic.

Yeah no reason to block your patch.  I'll think about it.

It's just that the 1st !MM_CP_DIRTY_ACCT check looks very hard to follow,
however indeed I don't have any better idea to rewrite it.

Then the patch looks good to me:

Reviewed-by: Peter Xu <peterx@redhat.com>

Thanks,

-- 
Peter Xu