On Fri, Aug 02, 2024, maobibo wrote:

On 2024/7/27 上午7:52, Sean Christopherson wrote:

quoted

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 (local)
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
  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
stage-2 PTEs, then its folio has already been marked dirty.