[PATCH v1] KVM: s390: Fall back to short-term pinning in MAP ioctl

[Jaehoon Kim <jhkim@linux.ibm.com>]

FOLL_LONGTERM pinning fails for some memory types, such as file-backed
guest memory. As a result, kvm_s390_adapter_map() returns -EINVAL and
irqfd adapter registration fails even though interrupt delivery could
still work via the existing non-atomic path.

When FOLL_LONGTERM pinning fails, verify that the page is accessible
using a short-term pin instead. If the short-term pin succeeds,
immediately unpin the page and return success. The non-atomic irqfd
path already performs short-term pinning for interrupt delivery, so
this restores the previous behavior for memory that cannot be pinned
long-term.

Fixes: adcd5b3e758b ("KVM: s390: Add map/unmap ioctl and clean mappings post-guest")
Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
---
 arch/s390/kvm/interrupt.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index 9e3e6b0d72ad..4b68e0cb51f6 100644
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -2520,8 +2520,19 @@ static int kvm_s390_adapter_map(struct kvm *kvm, unsigned int id, __u64 addr)
 	map->addr = host_addr;
 	map->page = pin_map_page(kvm, host_addr, FOLL_LONGTERM);
 	if (!map->page) {
-		ret = -EINVAL;
-		goto out;
+		/*
+		 * Long-term pinning may fail for memory types such as file-backed
+		 * memory. Check whether short-term pinning is possible so that the
+		 * non-atomic irqfd path can handle interrupt injection.
+		 */
+		map->page = pin_map_page(kvm, host_addr, 0);
+		if (!map->page) {
+			ret = -EINVAL;
+			goto out;
+		}
+		unpin_user_page(map->page);
+		kfree(map);
+		return 0;
 	}
 	spin_lock_irqsave(&adapter->maps_lock, flags);
 	if (adapter->nr_maps < MAX_S390_ADAPTER_MAPS) {
-- 
2.43.0