Re: [PATCH RFC 13/24] mm: merge parameters for change_protection()

Peter Xu
2019-01-24T05:22:46Z

On Mon, Jan 21, 2019 at 08:54:46AM -0500, Jerome Glisse wrote:
> On Mon, Jan 21, 2019 at 03:57:11PM +0800, Peter Xu wrote:
> > change_protection() was used by either the NUMA or mprotect() code,
> > there's one parameter for each of the callers (dirty_accountable and
> > prot_numa). Further, these parameters are passed along the calls:
> >
> > - change_protection_range()
> > - change_p4d_range()
> > - change_pud_range()
> > - change_pmd_range()
> > - ...
> >
> > Now we introduce a flag for change_protect() and all these helpers to
> > replace these parameters. Then we can avoid passing multiple parameters
> > multiple times along the way.
> >
> > More importantly, it'll greatly simplify the work if we want to
> > introduce any new parameters to change_protection(). In the follow up
> > patches, a new parameter for userfaultfd write protection will be
> > introduced.
> >
> > No functional change at all.
>
> There is one change i could spot and also something that looks wrong.
>
> >
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
>
> [...]
>
> > @@ -428,8 +431,7 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
> > dirty_accountable = vma_wants_writenotify(vma, vma->vm_page_prot);
> > vma_set_page_prot(vma);
> >
> > - change_protection(vma, start, end, vma->vm_page_prot,
> > - dirty_accountable, 0);
> > + change_protection(vma, start, end, vma->vm_page_prot, MM_CP_DIRTY_ACCT);
>
> Here you unconditionaly see the DIRTY_ACCT flag instead it should be
> something like:
>
> s/dirty_accountable/cp_flags
> if (vma_wants_writenotify(vma, vma->vm_page_prot))
> cp_flags = MM_CP_DIRTY_ACCT;
> else
> cp_flags = 0;
>
> change_protection(vma, start, end, vma->vm_page_prot, cp_flags);
>
> Or any equivalent construct.

Oops, thanks for spotting this... it was definitely wrong. I'll fix.

>
> > /*
> > * Private VM_LOCKED VMA becoming writable: trigger COW to avoid major
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 005291b9b62f..23d4bbd117ee 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -674,7 +674,7 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> > newprot = vm_get_page_prot(dst_vma->vm_flags);
> >
> > change_protection(dst_vma, start, start + len, newprot,
> > - !enable_wp, 0);
> > + enable_wp ? 0 : MM_CP_DIRTY_ACCT);
>
> We had a discussion in the past on that, i have not look at other
> patches but this seems wrong to me. MM_CP_DIRTY_ACCT is an
> optimization to keep a pte with write permission if it is dirty
> while my understanding is that you want to set write flag for pte
> unconditionaly.
>
> So maybe this patch that adds flag should be earlier in the serie
> so that you can add a flag to do that before introducing the UFD
> mwriteprotect_range() function.

I agree. I'm going to move the UFFDIO_WRITEPROTECT patch to the last
so I'll rearrange this part too so these lines will be removed in my
next version.

Thanks!

--
Peter Xu