s390/uv: Always return 0 from s390_wiggle_split_folio() if successful

JIRA: https://issues.redhat.com/browse/RHEL-58218

commit bd428b8
Author: David Hildenbrand <david@redhat.com>
Date:   Fri May 16 14:39:45 2025 +0200

    s390/uv: Always return 0 from s390_wiggle_split_folio() if successful

    Let's consistently return 0 if the operation was successful, and just
    detect ourselves whether splitting is required -- folio_test_large() is
    a cheap operation.

    Update the documentation.

    Should we simply always return -EAGAIN instead of 0, so we don't have
    to handle it in the caller? Not sure, staring at the documentation, this
    way looks a bit cleaner.

    Signed-off-by: David Hildenbrand <david@redhat.com>
    Link: https://lore.kernel.org/r/20250516123946.1648026-3-david@redhat.com
    Message-ID: <20250516123946.1648026-3-david@redhat.com>
    Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
    Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

Signed-off-by: Thomas Huth <thuth@redhat.com>

Author: huth

arch/s390/kernel/uv.c

@@ -324,34 +324,36 @@ static int make_folio_secure(struct mm_struct *mm, struct folio *folio, struct u
 }
 
 /**
- * s390_wiggle_split_folio() - try to drain extra references to a folio and optionally split.
+ * s390_wiggle_split_folio() - try to drain extra references to a folio and
+ *			       split the folio if it is large.
  * @mm:    the mm containing the folio to work on
  * @folio: the folio
- * @split: whether to split a large folio
  *
  * Context: Must be called while holding an extra reference to the folio;
  *          the mm lock should not be held.
- * Return: 0 if the folio was split successfully;
- *         -EAGAIN if the folio was not split successfully but another attempt
- *                 can be made, or if @split was set to false;
- *         -EINVAL in case of other errors. See split_folio().
+ * Return: 0 if the operation was successful;
+ *	   -EAGAIN if splitting the large folio was not successful,
+ *		   but another attempt can be made;
+ *	   -EINVAL in case of other folio splitting errors. See split_folio().
  */
-static int s390_wiggle_split_folio(struct mm_struct *mm, struct folio *folio, bool split)
+static int s390_wiggle_split_folio(struct mm_struct *mm, struct folio *folio)
 {
 	int rc;
 
 	lockdep_assert_not_held(&mm->mmap_lock);
 	folio_wait_writeback(folio);
 	lru_add_drain_all();
-	if (split) {
+
+	if (folio_test_large(folio)) {
 		folio_lock(folio);
 		rc = split_folio(folio);
 		folio_unlock(folio);
 
 		if (rc != -EBUSY)
 			return rc;
+		return -EAGAIN;
 	}
-	return -EAGAIN;
+	return 0;
 }
 
 int make_hva_secure(struct mm_struct *mm, unsigned long hva, struct uv_cb_header *uvcb)
@@ -394,7 +396,7 @@ int make_hva_secure(struct mm_struct *mm, unsigned long hva, struct uv_cb_header
 	mmap_read_unlock(mm);
 
 	if (rc == -E2BIG || rc == -EBUSY) {
-		rc = s390_wiggle_split_folio(mm, folio, rc == -E2BIG);
+		rc = s390_wiggle_split_folio(mm, folio);
 		if (!rc)
 			rc = -EAGAIN;
 	}