From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Hildenbrand Subject: Re: [PATCH 2/2] kvm: Use huge pages for DAX-backed files Date: Tue, 13 Nov 2018 10:36:13 +0100 Message-ID: <043a592d-6592-3053-15a0-68cc54a26deb@redhat.com> References: <20181109203921.178363-1-brho@google.com> <20181109203921.178363-3-brho@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, yu.c.zhang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org, linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org, x86-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, "H. Peter Anvin" , yi.z.zhang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org To: Barret Rhoden , Dan Williams , Dave Jiang , Ross Zwisler , Vishal Verma , Paolo Bonzini , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Thomas Gleixner , Ingo Molnar , Borislav Petkov Return-path: In-Reply-To: <20181109203921.178363-3-brho-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> Content-Language: en-US List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" List-Id: kvm.vger.kernel.org On 09.11.18 21:39, Barret Rhoden wrote: > This change allows KVM to map DAX-backed files made of huge pages with > huge mappings in the EPT/TDP. > > DAX pages are not PageTransCompound. The existing check is trying to > determine if the mapping for the pfn is a huge mapping or not. For > non-DAX maps, e.g. hugetlbfs, that means checking PageTransCompound. > For DAX, we can check the page table itself. > > Note that KVM already faulted in the page (or huge page) in the host's > page table, and we hold the KVM mmu spinlock (grabbed before checking > the mmu seq). I wonder if the KVM mmu spinlock is enough for walking (not KVM exclusive) host page tables. Can you elaborate? > > Signed-off-by: Barret Rhoden > --- > arch/x86/kvm/mmu.c | 34 ++++++++++++++++++++++++++++++++-- > 1 file changed, 32 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c > index cf5f572f2305..2df8c459dc6a 100644 > --- a/arch/x86/kvm/mmu.c > +++ b/arch/x86/kvm/mmu.c > @@ -3152,6 +3152,36 @@ static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn) > return -EFAULT; > } > > +static bool pfn_is_huge_mapped(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn) > +{ > + struct page *page = pfn_to_page(pfn); > + unsigned long hva, map_shift; > + > + if (!is_zone_device_page(page)) > + return PageTransCompoundMap(page); > + > + /* > + * DAX pages do not use compound pages. The page should have already > + * been mapped into the host-side page table during try_async_pf(), so > + * we can check the page tables directly. > + */ > + hva = gfn_to_hva(kvm, gfn); > + if (kvm_is_error_hva(hva)) > + return false; > + > + /* > + * Our caller grabbed the KVM mmu_lock with a successful > + * mmu_notifier_retry, so we're safe to walk the page table. > + */ > + map_shift = dev_pagemap_mapping_shift(hva, current->mm); You could get rid of that local variable map_shift. > + switch (map_shift) { > + case PMD_SHIFT: > + case PUD_SIZE: > + return true; > + } > + return false; > +} > + > static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu, > gfn_t *gfnp, kvm_pfn_t *pfnp, > int *levelp) > @@ -3168,7 +3198,7 @@ static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu, > */ > if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn) && > level == PT_PAGE_TABLE_LEVEL && > - PageTransCompoundMap(pfn_to_page(pfn)) && > + pfn_is_huge_mapped(vcpu->kvm, gfn, pfn) && > !mmu_gfn_lpage_is_disallowed(vcpu, gfn, PT_DIRECTORY_LEVEL)) { > unsigned long mask; > /* > @@ -5678,7 +5708,7 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm, > */ > if (sp->role.direct && > !kvm_is_reserved_pfn(pfn) && > - PageTransCompoundMap(pfn_to_page(pfn))) { > + pfn_is_huge_mapped(kvm, sp->gfn, pfn)) { > pte_list_remove(rmap_head, sptep); > need_tlb_flush = 1; > goto restart; > This looks surprisingly simple to me :) -- Thanks, David / dhildenb