Messages in this thread Patch in this message | | | Date | Wed, 28 Aug 2019 09:55:01 -0600 | From | Alex Williamson <> | Subject | Re: [PATCH v2] vfio/type1: avoid redundant PageReserved checking |
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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
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; }
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