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

[JAEHOON KIM]

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@xxxxxxxxxxxxx>
> > > ---
> > >   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.