[PATCH v6 05/14] KVM: Try using fast GUP to resolve read faults

[Anish Moorthy <amoorthy@google.com>]

hva_to_pfn_fast() currently just fails for faults where establishing
writable mappings is forbidden, which is unnecessary. Instead, try
getting the page without passing FOLL_WRITE. This allows the
aforementioned faults to (potentially) be resolved without falling back
to slow GUP.

Suggested-by: James Houghton <jthoughton@google.com>
Signed-off-by: Anish Moorthy <amoorthy@google.com>
---
 virt/kvm/kvm_main.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 88946d5d102b..725191333c4e 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2811,7 +2811,7 @@ static inline int check_user_page_hwpoison(unsigned long addr)
 }
 
 /*
- * The fast path to get the writable pfn which will be stored in @pfn,
+ * The fast path to get the pfn which will be stored in @pfn,
  * true indicates success, otherwise false is returned.  It's also the
  * only part that runs if we can in atomic context.
  */
@@ -2825,10 +2825,9 @@ static bool hva_to_pfn_fast(unsigned long addr, bool write_fault,
 	 * or the caller allows to map a writable pfn for a read fault
 	 * request.
 	 */
-	if (!(write_fault || writable))
-		return false;
+	unsigned int gup_flags = (write_fault || writable) ? FOLL_WRITE : 0;
 
-	if (get_user_page_fast_only(addr, FOLL_WRITE, page)) {
+	if (get_user_page_fast_only(addr, gup_flags, page)) {
 		*pfn = page_to_pfn(page[0]);
 
 		if (writable)
-- 
2.42.0.869.gea05f2083d-goog