On Wed, Jul 5, 2023 at 10:19 PM Zhi Wang [off-list ref] wrote:

On Tue,  4 Jul 2023 16:50:48 +0900
David Stevens [off-list ref] 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

                                              ^guarded

quoted

+      * be true if FOLL_GET is not set.
+      */
+     bool guarded_by_mmu_notifier;

It seems no one sets the guraded_by_mmu_notifier in this patch. Is
guarded_by_mmu_notifier always equal to !foll->FOLL_GET and set by the
caller of __kvm_follow_pfn()?

Yes, this is the case.

If yes, do we have to use FOLL_GET to resolve GFN associated with a tail page?
It seems gup can tolerate gup_flags without FOLL_GET, but it is more like a
temporary solution. I don't think it is a good idea to play tricks with
a temporary solution, more like we are abusing the toleration.

I'm not sure I understand what you're getting at. This series never
calls gup without FOLL_GET.

This series aims to provide kvm_follow_pfn as a unified API on top of
gup+follow_pte. Since one of the major clients of this API uses an mmu
notifier, it makes sense to support returning a pfn without taking a
reference. And we indeed need to do that for certain types of memory.

Is a flag like guarded_by_mmu_notifier (perhaps a better name) enough to
indicate a tail page?

What do you mean by to indicate a tail page? Do you mean to indicate
that the returned pfn refers to non-refcounted page? That's specified
by is_refcounted_page.

-David