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

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 7/20/26 6:22 PM, Jaehoon Kim wrote:
> 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;

This makes sense, but what happens later on the kvm_s390_adapter_unmap?

Won't we return -ENOENT because nothing was in the list?

Is it worth detecting/recognizing this case?  I could think of 2 ways...

1) re-doing the above test during unmap

2) stashing an entry in adapter->maps but with a new flag that indicates
it was approved for short-term pinning only (e.g. return NULL
immediately if a match is found during get_map_info() with this flag
set), and during kvm_s390_adapter_unmap() and
kvm_s390_unmap_all_adapters() just remove it from the list without doing
any unpin/page dirtying.

#2 might be a little more code but sounds cleaner?