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
