linux-kernel - Re: [PATCH v3 14/28] userfaultfd: wp: handle COW properly for uffd-wp

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Date:   Mon, 22 Apr 2019 10:54:02 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Maya Gokhale <gokhale2@...l.gov>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Martin Cracauer <cracauer@...s.org>, Shaohua Li <shli@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Marty McFadden <mcfadden8@...l.gov>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH v3 14/28] userfaultfd: wp: handle COW properly for uffd-wp

On Mon, Apr 22, 2019 at 08:20:10PM +0800, Peter Xu wrote:
> On Fri, Apr 19, 2019 at 11:02:53AM -0400, Jerome Glisse wrote:
> 
> [...]
> 
> > > > > +			if (uffd_wp_resolve) {
> > > > > +				/* If the fault is resolved already, skip */
> > > > > +				if (!pte_uffd_wp(*pte))
> > > > > +					continue;
> > > > > +				page = vm_normal_page(vma, addr, oldpte);
> > > > > +				if (!page || page_mapcount(page) > 1) {
> > > > > +					struct vm_fault vmf = {
> > > > > +						.vma = vma,
> > > > > +						.address = addr & PAGE_MASK,
> > > > > +						.page = page,
> > > > > +						.orig_pte = oldpte,
> > > > > +						.pmd = pmd,
> > > > > +						/* pte and ptl not needed */
> > > > > +					};
> > > > > +					vm_fault_t ret;
> > > > > +
> > > > > +					if (page)
> > > > > +						get_page(page);
> > > > > +					arch_leave_lazy_mmu_mode();
> > > > > +					pte_unmap_unlock(pte, ptl);
> > > > > +					ret = wp_page_copy(&vmf);
> > > > > +					/* PTE is changed, or OOM */
> > > > > +					if (ret == 0)
> > > > > +						/* It's done by others */
> > > > > +						continue;
> > > > 
> > > > This is wrong if ret == 0 you still need to remap the pte before
> > > > continuing as otherwise you will go to next pte without the page
> > > > table lock for the directory. So 0 case must be handled after
> > > > arch_enter_lazy_mmu_mode() below.
> > > > 
> > > > Sorry i should have catch that in previous review.
> > > 
> > > My fault to not have noticed it since the very beginning... thanks for
> > > spotting that.
> > > 
> > > I'm squashing below changes into the patch:
> > 
> > 
> > Well thinking of this some more i think you should use do_wp_page() and
> > not wp_page_copy() it would avoid bunch of code above and also you are
> > not properly handling KSM page or page in the swap cache. Instead of
> > duplicating same code that is in do_wp_page() it would be better to call
> > it here.
> 
> Yeah it makes sense to me.  Then here's my plan:
> 
> - I'll need to drop previous patch "export wp_page_copy" since then
>   it'll be not needed
> 
> - I'll introduce another patch to split current do_wp_page() and
>   introduce function "wp_page_copy_cont" (better suggestion on the
>   naming would be welcomed) which contains most of the wp handling
>   that'll be needed for change_pte_range() in this patch and isolate
>   the uffd handling:
> 
> static vm_fault_t do_wp_page(struct vm_fault *vmf)
> 	__releases(vmf->ptl)
> {
> 	struct vm_area_struct *vma = vmf->vma;
> 
> 	if (userfaultfd_pte_wp(vma, *vmf->pte)) {
> 		pte_unmap_unlock(vmf->pte, vmf->ptl);
> 		return handle_userfault(vmf, VM_UFFD_WP);
> 	}
> 
> 	return do_wp_page_cont(vmf);
> }
> 
> Then I can probably use do_wp_page_cont() in this patch.

Instead i would keep the do_wp_page name and do:
    static vm_fault_t do_userfaultfd_wp_page(struct vm_fault *vmf) {
        ... // what you have above
        return do_wp_page(vmf);
    }

Naming wise i think it would be better to keep do_wp_page() as
is.

Cheers,
Jérôme