Re: [PATCH v12 64/84] KVM: LoongArch: Mark "struct page" pfns dirty only in "slow" page fault path

[maobibo]

On 2024/8/3 上午3:32, Sean Christopherson wrote:
> On Fri, Aug 02, 2024, maobibo wrote:
> > On 2024/7/27 上午7:52, Sean Christopherson wrote:
> > > Mark pages/folios dirty only the slow page fault path, i.e. only when
> > > mmu_lock is held and the operation is mmu_notifier-protected, as marking a
> > > page/folio dirty after it has been written back can make some filesystems
> > > unhappy (backing KVM guests will such filesystem files is uncommon, and
> > > the race is minuscule, hence the lack of complaints).
> > >
> > > See the link below for details.
> > >
> > > Link: https://lore.kernel.org/all/cover.1683044162.git.lstoakes@xxxxxxxxx
> > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > ---
> > >    arch/loongarch/kvm/mmu.c | 18 ++++++++++--------
> > >    1 file changed, 10 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/loongarch/kvm/mmu.c b/arch/loongarch/kvm/mmu.c
> > > index 2634a9e8d82c..364dd35e0557 100644
> > > --- a/arch/loongarch/kvm/mmu.c
> > > +++ b/arch/loongarch/kvm/mmu.c
> > > @@ -608,13 +608,13 @@ static int kvm_map_page_fast(struct kvm_vcpu *vcpu, unsigned long gpa, bool writ
> > >    		if (kvm_pte_young(changed))
> > >    			kvm_set_pfn_accessed(pfn);
> > > -		if (kvm_pte_dirty(changed)) {
> > > -			mark_page_dirty(kvm, gfn);
> > > -			kvm_set_pfn_dirty(pfn);
> > > -		}
> > >    		if (page)
> > >    			put_page(page);
> > >    	}
> > > +
> > > +	if (kvm_pte_dirty(changed))
> > > +		mark_page_dirty(kvm, gfn);
> > > +
> > >    	return ret;
> > >    out:
> > >    	spin_unlock(&kvm->mmu_lock);
> > > @@ -915,12 +915,14 @@ static int kvm_map_page(struct kvm_vcpu *vcpu, unsigned long gpa, bool write)
> > >    	else
> > >    		++kvm->stat.pages;
> > >    	kvm_set_pte(ptep, new_pte);
> > > -	spin_unlock(&kvm->mmu_lock);
> > > -	if (prot_bits & _PAGE_DIRTY) {
> > > -		mark_page_dirty_in_slot(kvm, memslot, gfn);
> > > +	if (writeable)
> > Is it better to use write or (prot_bits & _PAGE_DIRTY) here?  writable is
> > pte permission from function hva_to_pfn_slow(), write is fault action.
>
> Marking folios dirty in the slow/full path basically necessitates marking the
> folio dirty if KVM creates a writable SPTE, as KVM won't mark the folio dirty
> if/when _PAGE_DIRTY is set.
>
> Practically speaking, I'm 99.9% certain it doesn't matter.  The folio is marked
> dirty by core MM when the folio is made writable, and cleaning the folio triggers
> an mmu_notifier invalidation.  I.e. if the page is mapped writable in KVM's
yes, it is. Thanks for the explanation. kvm_set_pfn_dirty() can be put 
only in slow page fault path. I only concern with fault type, read fault 
type can set pte entry writable however not _PAGE_DIRTY at stage-2 mmu 
table.

> stage-2 PTEs, then its folio has already been marked dirty.
Considering one condition although I do not know whether it exists 
actually. user mode VMM writes the folio with hva address firstly, then 
VCPU thread *reads* the folio. With primary mmu table, pte entry is 
writable and _PAGE_DIRTY is set, with secondary mmu table(state-2 PTE 
table), it is pte_none since the filio is accessed at first time, so 
there will be slow page fault path for stage-2 mmu page table filling.

Since it is read fault, stage-2 PTE will be created with 
_PAGE_WRITE(coming from function hva_to_pfn_slow()), however _PAGE_DIRTY 
is not set. Do we need call kvm_set_pfn_dirty() at this situation?

Regards
Bibo Mao