linux-kernel - Re: [PATCH v5 11/13] KVM: Zap existing KVM mappings when pages changed in the private fd

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Message-ID: <YkNcmGsOw4MThaym@google.com>
Date:   Tue, 29 Mar 2022 19:23:04 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, qemu-devel@...gnu.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com
Subject: Re: [PATCH v5 11/13] KVM: Zap existing KVM mappings when pages
 changed in the private fd

On Thu, Mar 10, 2022, Chao Peng wrote:
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 67349421eae3..52319f49d58a 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -841,8 +841,43 @@ static int kvm_init_mmu_notifier(struct kvm *kvm)
>  #endif /* CONFIG_MMU_NOTIFIER && KVM_ARCH_WANT_MMU_NOTIFIER */
>  
>  #ifdef CONFIG_MEMFILE_NOTIFIER
> +static void kvm_memfile_notifier_handler(struct memfile_notifier *notifier,
> +					 pgoff_t start, pgoff_t end)
> +{
> +	int idx;
> +	struct kvm_memory_slot *slot = container_of(notifier,
> +						    struct kvm_memory_slot,
> +						    notifier);
> +	struct kvm_gfn_range gfn_range = {
> +		.slot		= slot,
> +		.start		= start - (slot->private_offset >> PAGE_SHIFT),
> +		.end		= end - (slot->private_offset >> PAGE_SHIFT),
> +		.may_block 	= true,
> +	};
> +	struct kvm *kvm = slot->kvm;
> +
> +	gfn_range.start = max(gfn_range.start, slot->base_gfn);
> +	gfn_range.end = min(gfn_range.end, slot->base_gfn + slot->npages);
> +
> +	if (gfn_range.start >= gfn_range.end)
> +		return;
> +
> +	idx = srcu_read_lock(&kvm->srcu);
> +	KVM_MMU_LOCK(kvm);
> +	kvm_unmap_gfn_range(kvm, &gfn_range);
> +	kvm_flush_remote_tlbs(kvm);

This should check the result of kvm_unmap_gfn_range() and flush only if necessary.

kvm->mmu_notifier_seq needs to be incremented, otherwise KVM will incorrectly
install a SPTE if the mapping is zapped between retrieving the pfn in faultin and
installing it after acquire mmu_lock.


> +	KVM_MMU_UNLOCK(kvm);
> +	srcu_read_unlock(&kvm->srcu, idx);
> +}
> +
> +static struct memfile_notifier_ops kvm_memfile_notifier_ops = {
> +	.invalidate = kvm_memfile_notifier_handler,
> +	.fallocate = kvm_memfile_notifier_handler,
> +};
> +
>  static inline int kvm_memfile_register(struct kvm_memory_slot *slot)
>  {
> +	slot->notifier.ops = &kvm_memfile_notifier_ops;
>  	return memfile_register_notifier(file_inode(slot->private_file),
>  					 &slot->notifier,
>  					 &slot->pfn_ops);
> @@ -1963,6 +1998,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
>  	new->private_file = file;
>  	new->private_offset = mem->flags & KVM_MEM_PRIVATE ?
>  			      region_ext->private_offset : 0;
> +	new->kvm = kvm;
>  
>  	r = kvm_set_memslot(kvm, old, new, change);
>  	if (!r)
> -- 
> 2.17.1
>