Subject: [PATCH v7 4/4] arm: dirty page logging 2nd stage page fault handling support

From: [email-masked] (Christoffer Dall)
Date: Sun, 8 Jun 2014 14:05:30 +0200

On Tue, Jun 03, 2014 at 04:19:27PM -0700, Mario Smarduch wrote:
> This patch adds support for handling 2nd stage page faults during migration,
> it disables faulting in huge pages, and disolves huge pages to page tables.

s/disolves/dissolves/g

> In case migration is canceled huge pages will be used again.
> 
> Signed-off-by: Mario Smarduch <[email-masked]>
> ---
>  arch/arm/kvm/mmu.c |   36 ++++++++++++++++++++++++++++++++++--
>  1 file changed, 34 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 1c546c9..aca4fbf 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -966,6 +966,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	struct kvm_mmu_memory_cache *memcache = &vcpu->arch.mmu_page_cache;
>  	struct vm_area_struct *vma;
>  	pfn_t pfn;
> +	/* Get logging status, if dirty_bitmap is not NULL then logging is on */
> +	bool logging_active = !!memslot->dirty_bitmap;

>  
>  	write_fault = kvm_is_write_fault(kvm_vcpu_get_hsr(vcpu));
>  	if (fault_status == FSC_PERM && !write_fault) {
> @@ -1019,10 +1021,16 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	spin_lock(&kvm->mmu_lock);
>  	if (mmu_notifier_retry(kvm, mmu_seq))
>  		goto out_unlock;
> -	if (!hugetlb && !force_pte)
> +
> +	/* When logging don't spend cycles to check for huge pages */

drop the comment: either explain the entire clause (which would be too
long) or don't explain anything.

> +	if (!hugetlb && !force_pte && !logging_active)

instead of having all this, can't you just change 

if (is_vm_hugetlb_page(vma)) to
if (is_vm_hugetlb_page(vma) && !logging_active)

then you're also not mucking around with the gfn etc.

>  		hugetlb = transparent_hugepage_adjust(&pfn, &fault_ipa);
>  
> -	if (hugetlb) {
> +	/*
> +	 * Force all not present/perm faults to PTE handling, address both
> +	 * PMD and PTE faults
> +	 */

I don't understand this comment?  In which case does this apply?

> +	if (hugetlb && !logging_active) {
>  		pmd_t new_pmd = pfn_pmd(pfn, PAGE_S2);
>  		new_pmd = pmd_mkhuge(new_pmd);
>  		if (writable) {
> @@ -1034,6 +1042,22 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	} else {
>  		pte_t new_pte = pfn_pte(pfn, PAGE_S2);
>  		if (writable) {
> +			/*
> +			 * If pmd is  mapping a huge page then clear it and let
> +			 * stage2_set_pte() create a pte table. At the sametime
> +			 * you write protect the pte (PAGE_S2 pgprot_t).
> +			 */
> +			if (logging_active) {
> +				pmd_t *pmd;
> +				if (hugetlb) {
> +					pfn += pte_index(fault_ipa);
> +					gfn = fault_ipa >> PAGE_SHIFT;
> +					new_pte = pfn_pte(pfn, PAGE_S2);
> +				}
> +				pmd = stage2_get_pmd(kvm, NULL, fault_ipa);
> +				if (pmd && kvm_pmd_huge(*pmd))
> +					clear_pmd_entry(kvm, pmd, fault_ipa);
> +			}

now instead of all this, you just need to check for kvm_pmd_huge() in
stage2_set_pte() and if that's true, you clear it, and then then install
your new pte.

>  			kvm_set_s2pte_writable(&new_pte);
>  			kvm_set_pfn_dirty(pfn);
>  		}
> @@ -1041,6 +1065,14 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  		ret = stage2_set_pte(kvm, memcache, fault_ipa, &new_pte, false);
>  	}
>  
> +	/*
> +	 * Log the dirty page in dirty_bitmap[], call regardless if logging is
> +	 * disabled or enabled both cases handled safely.
> +	 * TODO: for larger page size mark mulitple dirty page bits for each
> +	 *       4k page.
> +	 */
> +	if (writable)
> +		mark_page_dirty(kvm, gfn);

what if you just faulted in a page on a read which wasn't present
before but it happens to belong to a writeable memslot, is that page
then dirty? hmmm.

>  
>  out_unlock:
>  	spin_unlock(&kvm->mmu_lock);
> -- 
> 1.7.9.5
> 

Thanks,
-Christoffer