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

[JAEHOON KIM <jhkim@linux.ibm.com>]

On 7/22/2026 12:07 PM, Christian Borntraeger wrote:
>
>
> Am 22.07.26 um 17:51 schrieb Matthew Rosato:
>> 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?
>>
>
> When we are at it, can we also fix 
> Documentation/virt/kvm/devices/s390_flic.rst ?
>
> To me it looks like qemu does not check the error for unmapping, but 
> yes, maybe 2 is
> the right thing to do.

I agree that option #2 is the cleaner approach as well. I'll also update the
documentation.

Thanks,
Jaehoon.