s390/uv: Split large folios in gmap_make_secure()

While s390x makes sure to never have PMD-mapped THP in processes that use
KVM -- by remapping them using PTEs in
thp_split_walk_pmd_entry()->split_huge_pmd() -- there is still the
possibility of having PTE-mapped THPs (large folios) mapped into guest
memory.

This would happen if user space allocates memory before calling
KVM_CREATE_VM (which would call s390_enable_sie()). With upstream QEMU,
this currently doesn't happen, because guest memory is setup and
conditionally preallocated after KVM_CREATE_VM.

Could it happen with shmem/file-backed memory when another process
allocated memory in the pagecache? Likely, although currently not a
common setup.

Trying to split any PTE-mapped large folios sounds like the right and
future-proof thing to do here. So let's call split_folio() and handle the
return values accordingly.
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20240508182955.358628-4-david@redhat.comSigned-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/kernel/uv.c

@@ -338,11 +338,10 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 		goto out;
 	if (pte_present(*ptep) && !(pte_val(*ptep) & _PAGE_INVALID) && pte_write(*ptep)) {
 		folio = page_folio(pte_page(*ptep));
-		rc = -EINVAL;
-		if (folio_test_large(folio))
-			goto unlock;
 		rc = -EAGAIN;
-		if (folio_trylock(folio)) {
+		if (folio_test_large(folio)) {
+			rc = -E2BIG;
+		} else if (folio_trylock(folio)) {
 			if (should_export_before_import(uvcb, gmap->mm))
 				uv_convert_from_secure(PFN_PHYS(folio_pfn(folio)));
 			rc = make_folio_secure(folio, uvcb);
@@ -353,15 +352,35 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 		 * Once we drop the PTL, the folio may get unmapped and
 		 * freed immediately. We need a temporary reference.
 		 */
-		if (rc == -EAGAIN)
+		if (rc == -EAGAIN || rc == -E2BIG)
 			folio_get(folio);
 	}
-unlock:
 	pte_unmap_unlock(ptep, ptelock);
 out:
 	mmap_read_unlock(gmap->mm);
 
 	switch (rc) {
+	case -E2BIG:
+		folio_lock(folio);
+		rc = split_folio(folio);
+		folio_unlock(folio);
+		folio_put(folio);
+
+		switch (rc) {
+		case 0:
+			/* Splitting succeeded, try again immediately. */
+			goto again;
+		case -EAGAIN:
+			/* Additional folio references. */
+			if (drain_lru(&drain_lru_called))
+				goto again;
+			return -EAGAIN;
+		case -EBUSY:
+			/* Unexpected race. */
+			return -EAGAIN;
+		}
+		WARN_ON_ONCE(1);
+		return -ENXIO;
 	case -EAGAIN:
 		/*
 		 * If we are here because the UVC returned busy or partial