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

[wangyanan (Y)]

On 2021/1/13 23:51, Will Deacon wrote:
> 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?
Yes, it's more concise.