Re: [PATCH v2 3/3] KVM: arm64: Mark the page dirty only if the fault is handled successfully

[Will Deacon]

On Wed, Dec 16, 2020 at 08:28:44PM +0800, Yanan Wang wrote:
> We now mark the page dirty and set the bitmap before calling fault handlers
> in user_mem_abort(), and we might end up having spurious dirty pages if
> update of permissions or mapping has failed.
> So, mark the page dirty only if the fault is handled successfully.
> 
> Let the guest directly enter again but not return to userspace if we were
> trying to recreate the same mapping or only change access permissions
> with BBM, which is not permitted in the mapping path.
> 
> Signed-off-by: Yanan Wang <wangyanan55@xxxxxxxxxx>
> ---
>  arch/arm64/kvm/mmu.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 75814a02d189..72e516a10914 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -879,11 +879,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	if (vma_pagesize == PAGE_SIZE && !force_pte)
>  		vma_pagesize = transparent_hugepage_adjust(memslot, hva,
>  							   &pfn, &fault_ipa);
> -	if (writable) {
> +	if (writable)
>  		prot |= KVM_PGTABLE_PROT_W;
> -		kvm_set_pfn_dirty(pfn);
> -		mark_page_dirty(kvm, gfn);
> -	}
>  
>  	if (fault_status != FSC_PERM && !device)
>  		clean_dcache_guest_page(pfn, vma_pagesize);
> @@ -911,6 +908,19 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  					     memcache);
>  	}
>  
> +	/* Mark the page dirty only if the fault is handled successfully */
> +	if (writable && !ret) {
> +		kvm_set_pfn_dirty(pfn);
> +		mark_page_dirty(kvm, gfn);
> +	}
> +
> +	/* Let the guest directly enter again if we were trying to recreate the
> +	 * same mapping or only change access permissions with BBM, which is not
> +	 * permitted in the mapping path.
> +	 */
> +	if (ret == -EAGAIN)
> +		ret = 0;

Maybe just 'return ret != -EAGAIN ? ret : 0;' at the end of the function?

Will