KVM: MMU: track dirty page in speculative path properly
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From: Marcelo Tosatti on 13 Jul 2010 18:10 On Tue, Jul 13, 2010 at 05:45:27PM +0800, Xiao Guangrong wrote: > In speculative path, the page is not real write-access, no need mark it > dirty, so clear dirty bit in this path and later examine this bit when > we release the page > > Signed-off-by: Xiao Guangrong <xiaoguangrong(a)cn.fujitsu.com> > --- > arch/x86/kvm/mmu.c | 24 +++++++++++------------- > 1 files changed, 11 insertions(+), 13 deletions(-) Unfortunately all pages that kvm creates translations for are marked dirty due to get_user_pages(w=1), except KSM which makes them read-only later. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Marcelo Tosatti on 14 Jul 2010 08:50 On Wed, Jul 14, 2010 at 09:24:22AM +0800, Xiao Guangrong wrote: > > > Marcelo Tosatti wrote: > > On Tue, Jul 13, 2010 at 05:45:27PM +0800, Xiao Guangrong wrote: > >> In speculative path, the page is not real write-access, no need mark it > >> dirty, so clear dirty bit in this path and later examine this bit when > >> we release the page > >> > >> Signed-off-by: Xiao Guangrong <xiaoguangrong(a)cn.fujitsu.com> > >> --- > >> arch/x86/kvm/mmu.c | 24 +++++++++++------------- > >> 1 files changed, 11 insertions(+), 13 deletions(-) > > > > Unfortunately all pages that kvm creates translations for are marked > > dirty due to get_user_pages(w=1), except KSM which makes them read-only > > later. > > Marcelo, i have looked into get_user_pages() function, but not catch where > to make page dirty, could you point it out for me? :-) See set_page_dirty call in mm/memory.c::follow_page. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Marcelo Tosatti on 14 Jul 2010 10:10 On Wed, Jul 14, 2010 at 09:18:58PM +0800, Xiao Guangrong wrote: > > > Marcelo Tosatti wrote: > > On Wed, Jul 14, 2010 at 09:24:22AM +0800, Xiao Guangrong wrote: > >> > >> Marcelo Tosatti wrote: > >>> On Tue, Jul 13, 2010 at 05:45:27PM +0800, Xiao Guangrong wrote: > >>>> In speculative path, the page is not real write-access, no need mark it > >>>> dirty, so clear dirty bit in this path and later examine this bit when > >>>> we release the page > >>>> > >>>> Signed-off-by: Xiao Guangrong <xiaoguangrong(a)cn.fujitsu.com> > >>>> --- > >>>> arch/x86/kvm/mmu.c | 24 +++++++++++------------- > >>>> 1 files changed, 11 insertions(+), 13 deletions(-) > >>> Unfortunately all pages that kvm creates translations for are marked > >>> dirty due to get_user_pages(w=1), except KSM which makes them read-only > >>> later. > >> Marcelo, i have looked into get_user_pages() function, but not catch where > >> to make page dirty, could you point it out for me? :-) > > > > See set_page_dirty call in mm/memory.c::follow_page. > > Yeah, you are right, and i want to use another way to do it since track dirty bit > is too heavy, also it's dangerous if we miss to set page dirty. > > How about just track access bit for speculative path, we set page both accessed and > dirty(if it's writable) only if the access bit is set? A useful thing to do would be to allow read-only mappings, in the fault path (Lai sent a few patches in that direction sometime ago but there was no follow up). So in the case of a read-only fault from the guest, you'd inform get_user_pages() that read-only access is acceptable (so swapcache pages can be mapped, or qemu can mprotect(PROT_READ) guest memory). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Marcelo Tosatti on 15 Jul 2010 14:00
On Thu, Jul 15, 2010 at 03:44:48PM +0800, Xiao Guangrong wrote: > > > Marcelo Tosatti wrote: > > >> How about just track access bit for speculative path, we set page both accessed and > >> dirty(if it's writable) only if the access bit is set? > > > > A useful thing to do would be to allow read-only mappings, in the fault > > path (Lai sent a few patches in that direction sometime ago but there > > was no follow up). > > > > So in the case of a read-only fault from the guest, you'd inform > > get_user_pages() that read-only access is acceptable (so swapcache pages > > can be mapped, or qemu can mprotect(PROT_READ) guest memory). > > > > Yeah, it's a great work, i guess Lai will post the new version soon. > > And, even we do this, i think the page dirty track is still needed, right? > Then, how about my new idea to track page dirty for speculative path, just > as below draft patch does: > > @@ -687,10 +687,11 @@ static void drop_spte(struct kvm *kvm, u64 *sptep, u64 new_spte) > if (!is_rmap_spte(old_spte)) > return; > pfn = spte_to_pfn(old_spte); > - if (old_spte & shadow_accessed_mask) > + if (old_spte & shadow_accessed_mask) { > kvm_set_pfn_accessed(pfn); > - if (is_writable_pte(old_spte)) > - kvm_set_pfn_dirty(pfn); > + if (is_writable_pte(old_spte)) > + kvm_set_pfn_dirty(pfn); > + } > rmap_remove(kvm, sptep); > } > > @@ -1920,8 +1921,11 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep, > * demand paging). > */ > spte = shadow_base_present_pte | shadow_dirty_mask; > - if (!speculative) > + if (!speculative) { > spte |= shadow_accessed_mask; > + if (is_writable_pte(*sptep)) > + kvm_set_pfn_dirty(pfn); > + } > if (!dirty) > pte_access &= ~ACC_WRITE_MASK; > if (pte_access & ACC_EXEC_MASK) > > It uses access bit to track both page accessed and page dirty, and it's rather cheap... Xiao, I don't understand it. What are you trying to achieve? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |