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Date:   Thu, 29 Aug 2019 11:06:01 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     Ben Luo <luoben@linux.alibaba.com>
Cc:     cohuck@redhat.com, linux-kernel@vger.kernel.org,
        Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH v2] vfio/type1: avoid redundant PageReserved checking
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On Fri, 30 Aug 2019 00:58:22 +0800
Ben Luo <luoben@linux.alibaba.com> wrote:

> 在 2019/8/28 下午11:55, Alex Williamson 写道:
> > On Wed, 28 Aug 2019 12:28:04 +0800
> > Ben Luo <luoben@linux.alibaba.com> wrote:
> >  
> >> currently, if the page is not a tail of compound page, it will be
> >> checked twice for the same thing.
> >>
> >> Signed-off-by: Ben Luo <luoben@linux.alibaba.com>
> >> ---
> >>   drivers/vfio/vfio_iommu_type1.c | 3 +--
> >>   1 file changed, 1 insertion(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> >> index 054391f..d0f7346 100644
> >> --- a/drivers/vfio/vfio_iommu_type1.c
> >> +++ b/drivers/vfio/vfio_iommu_type1.c
> >> @@ -291,11 +291,10 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
> >>   static bool is_invalid_reserved_pfn(unsigned long pfn)
> >>   {
> >>   	if (pfn_valid(pfn)) {
> >> -		bool reserved;
> >>   		struct page *tail = pfn_to_page(pfn);
> >>   		struct page *head = compound_head(tail);
> >> -		reserved = !!(PageReserved(head));
> >>   		if (head != tail) {
> >> +			bool reserved = PageReserved(head);
> >>   			/*
> >>   			 * "head" is not a dangling pointer
> >>   			 * (compound_head takes care of that)  
> > Thinking more about this, the code here was originally just a copy of
> > kvm_is_mmio_pfn() which was simplified in v3.12 with the commit below.
> > Should we instead do the same thing here?  Thanks,
> >
> > Alex  
> ok, and kvm_is_mmio_pfn() has also been updated since then, I will take 
> a look at that and compose a new patch

I'm not sure if the further updates are quite as relevant for vfio, but
appreciate your review of them.  Thanks,

Alex

> >
> > commit 11feeb498086a3a5907b8148bdf1786a9b18fc55
> > Author: Andrea Arcangeli <aarcange@redhat.com>
> > Date:   Thu Jul 25 03:04:38 2013 +0200
> >
> >      kvm: optimize away THP checks in kvm_is_mmio_pfn()
> >      
> >      The checks on PG_reserved in the page structure on head and tail pages
> >      aren't necessary because split_huge_page wouldn't transfer the
> >      PG_reserved bit from head to tail anyway.
> >      
> >      This was a forward-thinking check done in the case PageReserved was
> >      set by a driver-owned page mapped in userland with something like
> >      remap_pfn_range in a VM_PFNMAP region, but using hugepmds (not
> >      possible right now). It was meant to be very safe, but it's overkill
> >      as it's unlikely split_huge_page could ever run without the driver
> >      noticing and tearing down the hugepage itself.
> >      
> >      And if a driver in the future will really want to map a reserved
> >      hugepage in userland using an huge pmd it should simply take care of
> >      marking all subpages reserved too to keep KVM safe. This of course
> >      would require such a hypothetical driver to tear down the huge pmd
> >      itself and splitting the hugepage itself, instead of relaying on
> >      split_huge_page, but that sounds very reasonable, especially
> >      considering split_huge_page wouldn't currently transfer the reserved
> >      bit anyway.
> >      
> >      Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> >      Signed-off-by: Gleb Natapov <gleb@redhat.com>
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index d2836788561e..0fc25aed79a8 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -102,28 +102,8 @@ static bool largepages_enabled = true;
> >   
> >   bool kvm_is_mmio_pfn(pfn_t pfn)
> >   {
> > -       if (pfn_valid(pfn)) {
> > -               int reserved;
> > -               struct page *tail = pfn_to_page(pfn);
> > -               struct page *head = compound_trans_head(tail);
> > -               reserved = PageReserved(head);
> > -               if (head != tail) {
> > -                       /*
> > -                        * "head" is not a dangling pointer
> > -                        * (compound_trans_head takes care of that)
> > -                        * but the hugepage may have been splitted
> > -                        * from under us (and we may not hold a
> > -                        * reference count on the head page so it can
> > -                        * be reused before we run PageReferenced), so
> > -                        * we've to check PageTail before returning
> > -                        * what we just read.
> > -                        */
> > -                       smp_rmb();
> > -                       if (PageTail(tail))
> > -                               return reserved;
> > -               }
> > -               return PageReserved(tail);
> > -       }
> > +       if (pfn_valid(pfn))
> > +               return PageReserved(pfn_to_page(pfn));
> >   
> >          return true;
> >   }