On 2024/8/6 上午7:22, Sean Christopherson wrote:
On Sat, Aug 03, 2024, maobibo wrote:
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?
If KVM doesn't mark the folio dirty when the stage-2 _PAGE_DIRTY flag is set,
i.e. as proposed in this series, then yes, KVM needs to call kvm_set_pfn_dirty()
even though the VM hasn't (yet) written to the memory. In practice, KVM calling
kvm_set_pfn_dirty() is redundant the majority of the time, as the stage-1 PTE
will have _PAGE_DIRTY set, and that will get propagated to the folio when the
primary MMU does anything relevant with the PTE. And for file systems that care
about writeback, odds are very good that the folio was marked dirty even earlier,
when MM invoked vm_operations_struct.page_mkwrite().
The reason I am pushing to have all architectures mark pages/folios dirty in the
slow page fault path is that a false positive (marking a folio dirty without the
folio ever being written in _any_ context since the last pte_mkclean()) is rare,
and at worst results an unnecessary writeback. On the other hand, marking folios
It does not influence the result. At worst there is one unnecessary
kvm_set_pfn_dirty() before the last pte_mkclean(). That is ok for me,
and thanks for your detailed explanation.
dirty in fast page fault handlers (or anywhere else that isn't protected by
mmu_notifiers) is technically unsafe.
yeap, moving marking folios dirty to slow fault handler makes logic
clear and simple here, and technically safer.
Regards
Bibo Mao
In other words, the intent is to sacrifice accuracy to improve stability/robustness,
because the vast majority of time the loss in accuracy has no effect, and the worst
case scenario is that the kernel does I/O that wasn't necessary.
