mm: extend FAULT_FLAG_UNSHARE support to anything in a COW mapping

Extend FAULT_FLAG_UNSHARE to break COW on anything mapped into a
COW (i.e., private writable) mapping and adjust the documentation
accordingly.

FAULT_FLAG_UNSHARE will now also break COW when encountering the shared
zeropage, a pagecache page, a PFNMAP, ... inside a COW mapping, by
properly replacing the mapped page/pfn by a private copy (an exclusive
anonymous page).

Note that only do_wp_page() needs care: hugetlb_wp() already handles
FAULT_FLAG_UNSHARE correctly. wp_huge_pmd()/wp_huge_pud() also handles it
correctly, for example, splitting the huge zeropage on FAULT_FLAG_UNSHARE
such that we can handle FAULT_FLAG_UNSHARE on the PTE level.

This change is a requirement for reliable long-term R/O pinning in
COW mappings.

Link: https://lkml.kernel.org/r/20221116102659.70287-9-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm_types.h

@@ -1039,9 +1039,9 @@ typedef struct {
  * @FAULT_FLAG_REMOTE: The fault is not for current task/mm.
  * @FAULT_FLAG_INSTRUCTION: The fault was during an instruction fetch.
  * @FAULT_FLAG_INTERRUPTIBLE: The fault can be interrupted by non-fatal signals.
- * @FAULT_FLAG_UNSHARE: The fault is an unsharing request to unshare (and mark
- *                      exclusive) a possibly shared anonymous page that is
- *                      mapped R/O.
+ * @FAULT_FLAG_UNSHARE: The fault is an unsharing request to break COW in a
+ *                      COW mapping, making sure that an exclusive anon page is
+ *                      mapped after the fault.
  * @FAULT_FLAG_ORIG_PTE_VALID: whether the fault has vmf->orig_pte cached.
  *                        We should only access orig_pte if this flag set.
  *
@@ -1066,7 +1066,7 @@ typedef struct {
  *
  * The combination FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE is illegal.
  * FAULT_FLAG_UNSHARE is ignored and treated like an ordinary read fault when
- * no existing R/O-mapped anonymous page is encountered.
+ * applied to mappings that are not COW mappings.
  */
 enum fault_flag {
 	FAULT_FLAG_WRITE =		1 << 0,

mm/memory.c

@@ -3431,10 +3431,6 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 		}
 		wp_page_reuse(vmf);
 		return VM_FAULT_WRITE;
-	} else if (unshare) {
-		/* No anonymous page -> nothing to do. */
-		pte_unmap_unlock(vmf->pte, vmf->ptl);
-		return 0;
 	}
 copy:
 	/*