On Wed, Jul 5, 2023 at 8:56 PM Yu Zhang [off-list ref] wrote:

On Tue, Jul 04, 2023 at 04:50:48PM +0900, David Stevens wrote:

quoted

From: David Stevens <redacted>

Make it so that __kvm_follow_pfn does not imply FOLL_GET. This allows
callers to resolve a gfn when the associated pfn has a valid struct page
that isn't being actively refcounted (e.g. tail pages of non-compound
higher order pages). For a caller to safely omit FOLL_GET, all usages of
the returned pfn must be guarded by a mmu notifier.

This also adds a is_refcounted_page out parameter to kvm_follow_pfn that
is set when the returned pfn has an associated struct page with a valid
refcount. Callers that don't pass FOLL_GET should remember this value
and use it to avoid places like kvm_is_ad_tracked_page that assume a
non-zero refcount.

Signed-off-by: David Stevens <redacted>
---
 include/linux/kvm_host.h | 10 ++++++
 virt/kvm/kvm_main.c      | 67 +++++++++++++++++++++-------------------
 virt/kvm/pfncache.c      |  2 +-
 3 files changed, 47 insertions(+), 32 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index ef2763c2b12e..a45308c7d2d9 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h

@@ -1157,6 +1157,9 @@ unsigned long gfn_to_hva_memslot_prot(struct kvm_memory_slot *slot, gfn_t gfn,
 void kvm_release_page_clean(struct page *page);
 void kvm_release_page_dirty(struct page *page);

+void kvm_set_page_accessed(struct page *page);
+void kvm_set_page_dirty(struct page *page);
+
 struct kvm_follow_pfn {
      const struct kvm_memory_slot *slot;
      gfn_t gfn;

@@ -1164,10 +1167,17 @@ struct kvm_follow_pfn {
      bool atomic;
      /* Allow a read fault to create a writeable mapping. */
      bool allow_write_mapping;
+     /*
+      * Usage of the returned pfn will be guared by a mmu notifier. Must
+      * be true if FOLL_GET is not set.
+      */
+     bool guarded_by_mmu_notifier;

And how? Any place to check the invalidate seq?

kvm_follow_pfn can't meaningfully validate the seq number, since the
mmu notifier locking is handled by the caller. This is more of a
sanity check that the API is being used properly, as proposed here
[1]. I did deviate from the proposal with a bool instead of some type
of integer, since the exact value of mmu_seq wouldn't be useful.

[1] https://lore.kernel.org/all/ZGvUsf7lMkrNDHuE@google.com/#t (local)

-David

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