s390/vmem: Support 2G page splitting for KASAN shadow freeing

Export split_pud_page() so it can be used from the vmem code and teach
modify_pud_table() to split PUD-sized mappings when only a subrange
needs to be removed.

If the range to be removed covers a full PUD-sized mapping, keep the
existing behavior: clear the PUD entry and free the backing large page
(for non-direct mappings). Otherwise, split the PUD-mapped page into
PMD mappings and let the walker handle the smaller ranges.

This is needed for KASAN early shadow removal support: memory hotplug
freeing the KASAN early shadow is the only expected caller that will
try to free 2G PUD-mapped regions of non-direct mappings.

Signed-off-by: Vasily Gorbik <[e-mail]>
Reviewed-by: Heiko Carstens <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>

arch/s390/include/asm/page.h

···
 	return CC_TRANSFORM(cc);
 }
 
+int split_pud_page(pud_t *pudp, unsigned long addr);
+
 /* Bits int the storage key */
 #define _PAGE_CHANGED		0x02	/* HW changed bit		*/
 #define _PAGE_REFERENCED	0x04	/* HW referenced bit		*/

arch/s390/mm/pageattr.c

···
 	return rc;
 }
 
-static int split_pud_page(pud_t *pudp, unsigned long addr)
+int split_pud_page(pud_t *pudp, unsigned long addr)
 {
 	unsigned long pmd_addr, prot;
 	pmd_t *pm_dir, *pmdp;

arch/s390/mm/vmem.c

···
 			if (pud_leaf(*pud)) {
 				if (IS_ALIGNED(addr, PUD_SIZE) &&
 				    IS_ALIGNED(next, PUD_SIZE)) {
+					if (!direct)
+						vmem_free_pages(pud_deref(*pud), get_order(PUD_SIZE), altmap);
 					pud_clear(pud);
 					pages++;
+					continue;
+				} else {
+					split_pud_page(pud, addr & PUD_MASK);
 				}
-				continue;
 			}
 		} else if (pud_none(*pud)) {
 			if (IS_ALIGNED(addr, PUD_SIZE) &&