s390/mm/fault: convert do_secure_storage_access() from follow_page() to folio_walk

Let's get rid of another follow_page() user and perform the conversion
under PTL: Note that this is also what follow_page_pte() ends up doing.

Unfortunately we cannot currently optimize out the additional reference,
because arch_make_folio_accessible() must be called with a raised refcount
to protect against concurrent conversion to secure.  We can just move the
arch_make_folio_accessible() under the PTL, like follow_page_pte() would.

We'll effectively drop the "writable" check implied by FOLL_WRITE:
follow_page_pte() would also not check that when calling
arch_make_folio_accessible(), so there is no good reason for doing that
here.

We'll lose the secretmem check from follow_page() as well, about which we
shouldn't really care.

Link: https://lkml.kernel.org/r/20240802155524.517137-10-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/s390/mm/fault.c

@@ -34,6 +34,7 @@
 #include <linux/uaccess.h>
 #include <linux/hugetlb.h>
 #include <linux/kfence.h>
+#include <linux/pagewalk.h>
 #include <asm/asm-extable.h>
 #include <asm/asm-offsets.h>
 #include <asm/ptrace.h>
@@ -492,9 +493,9 @@ void do_secure_storage_access(struct pt_regs *regs)
 	union teid teid = { .val = regs->int_parm_long };
 	unsigned long addr = get_fault_address(regs);
 	struct vm_area_struct *vma;
+	struct folio_walk fw;
 	struct mm_struct *mm;
 	struct folio *folio;
-	struct page *page;
 	struct gmap *gmap;
 	int rc;
 
@@ -536,15 +537,18 @@ void do_secure_storage_access(struct pt_regs *regs)
 		vma = find_vma(mm, addr);
 		if (!vma)
 			return handle_fault_error(regs, SEGV_MAPERR);
-		page = follow_page(vma, addr, FOLL_WRITE | FOLL_GET);
-		if (IS_ERR_OR_NULL(page)) {
+		folio = folio_walk_start(&fw, vma, addr, 0);
+		if (!folio) {
 			mmap_read_unlock(mm);
 			break;
 		}
-		folio = page_folio(page);
-		if (arch_make_folio_accessible(folio))
-			send_sig(SIGSEGV, current, 0);
+		/* arch_make_folio_accessible() needs a raised refcount. */
+		folio_get(folio);
+		rc = arch_make_folio_accessible(folio);
 		folio_put(folio);
+		folio_walk_end(&fw, vma);
+		if (rc)
+			send_sig(SIGSEGV, current, 0);
 		mmap_read_unlock(mm);
 		break;
 	case KERNEL_FAULT: