KVM: s390: do not take mmap_sem on dirty log query

Dirty log query can take a long time for huge guests.
Holding the mmap_sem for very long times  can cause some unwanted
latencies.
Turns out that we do not need to hold the mmap semaphore.
We hold the slots_lock for gfn->hva translation and walk the page
tables with that address, so no need to look at the VMAs. KVM also
holds a reference to the mm, which should prevent other things
going away. During the walk we take the necessary ptl locks.
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

arch/s390/kvm/kvm-s390.c

@@ -274,7 +274,6 @@ static void kvm_s390_sync_dirty_log(struct kvm *kvm,
 	unsigned long address;
 	struct gmap *gmap = kvm->arch.gmap;
 
-	down_read(&gmap->mm->mmap_sem);
 	/* Loop over all guest pages */
 	last_gfn = memslot->base_gfn + memslot->npages;
 	for (cur_gfn = memslot->base_gfn; cur_gfn <= last_gfn; cur_gfn++) {
@@ -283,7 +282,6 @@ static void kvm_s390_sync_dirty_log(struct kvm *kvm,
 		if (gmap_test_and_clear_dirty(address, gmap))
 			mark_page_dirty(kvm, cur_gfn);
 	}
-	up_read(&gmap->mm->mmap_sem);
 }
 
 /* Section: vm related */