KVM: MIPS: Mark "struct page" pfns dirty only in "slow" page fault path

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@gmail.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-70-seanjc@google.com>

Author: sean-jc

arch/mips/kvm/mmu.c

@@ -514,7 +514,6 @@ static int _kvm_mips_map_page_fast(struct kvm_vcpu *vcpu, unsigned long gpa,
 		set_pte(ptep, pte_mkdirty(*ptep));
 		pfn = pte_pfn(*ptep);
 		mark_page_dirty(kvm, gfn);
-		kvm_set_pfn_dirty(pfn);
 	}
 
 	if (out_entry)
@@ -628,7 +627,6 @@ static int kvm_mips_map_page(struct kvm_vcpu *vcpu, unsigned long gpa,
 		if (write_fault) {
 			prot_bits |= __WRITEABLE;
 			mark_page_dirty(kvm, gfn);
-			kvm_set_pfn_dirty(pfn);
 		}
 	}
 	entry = pfn_pte(pfn, __pgprot(prot_bits));
@@ -642,6 +640,9 @@ static int kvm_mips_map_page(struct kvm_vcpu *vcpu, unsigned long gpa,
 	if (out_buddy)
 		*out_buddy = *ptep_buddy(ptep);
 
+	if (writeable)
+		kvm_set_pfn_dirty(pfn);
+
 	spin_unlock(&kvm->mmu_lock);
 	kvm_release_pfn_clean(pfn);
 	kvm_set_pfn_accessed(pfn);