The upcoming mem_fault_nowait commits will make it so that, when the
relevant cap is enabled, hva_to_pfn will return after calling
hva_to_pfn_fast without ever attempting to pin memory via
hva_to_pfn_slow.
hva_to_pfn_fast currently just fails for read-only faults. However,
there doesn't seem to be a reason that we can't just try pinning the
page without FOLL_WRITE instead of immediately falling back to slow-GUP.
This commit implements that behavior.
Suggested-by: James Houghton <jthoughton@xxxxxxxxxx>
Signed-off-by: Anish Moorthy <amoorthy@xxxxxxxxxx>
---
virt/kvm/kvm_main.c | 22 ++++++++++++----------
1 file changed, 12 insertions(+), 10 deletions(-)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d255964ec331e..dae5f48151032 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2479,7 +2479,7 @@ 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.
*/
@@ -2487,16 +2487,18 @@ static bool hva_to_pfn_fast(unsigned long addr, bool write_fault,
bool *writable, kvm_pfn_t *pfn)
{
struct page *page[1];
+ bool found_by_fast_gup =
+ get_user_page_fast_only(
+ addr,
+ /*
+ * Fast pin a writable pfn only if it is a write fault request
+ * or the caller allows to map a writable pfn for a read fault
+ * request.
+ */
+ (write_fault || writable) ? FOLL_WRITE : 0,
+ page);
- /*
- * Fast pin a writable pfn only if it is a write fault request
- * or the caller allows to map a writable pfn for a read fault
- * request.
- */
- if (!(write_fault || writable))
- return false;
-
- if (get_user_page_fast_only(addr, FOLL_WRITE, page)) {
+ if (found_by_fast_gup) {
*pfn = page_to_pfn(page[0]);
if (writable)
--
2.39.1.581.gbfd45094c4-goog
