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

[Mario Smarduch <m.smarduch@samsung.com>]

On 06/08/2014 05:05 AM, Christoffer Dall wrote:
> 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
Will do.
> 
>> In case migration is canceled huge pages will be used again.
>>
>> Signed-off-by: Mario Smarduch <m.smarduch@samsung.com>
>> ---
>>  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.
> 
Ok.
>> +	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.

I didn't want to modify this function too much, but if that's ok that 
simplifies things a lot.

> 
>>  		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?
> 
The cases I see here -
- huge page permission fault is forced into page table code while logging
- pte permission/not present handled by page table code as before.
>> +	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.

Yes this really simplifies things!

> 
>>  			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.
> 
A bug, must also check if it was a write fault not just that we're dealing with
a writable region. This one could be pretty bad on performance, not to mention
in accurate. It will be interesting to see new test results, glad you caught
that.

Thanks,
  Mario.
> 
>>  
>>  out_unlock:
>>  	spin_unlock(&kvm->mmu_lock);
>> -- 
>> 1.7.9.5
>>
> 
> Thanks,
> -Christoffer
>