Commit 8d6a0ac0 authored by David Hildenbrand's avatar David Hildenbrand Committed by Andrew Morton
Browse files

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.com


Signed-off-by: default avatarDavid Hildenbrand <david@redhat.com>
Reviewed-by: default avatarVlastimil Babka <vbabka@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent aea06577

include/linux/mm_types.h

+4 −4
Original line number Diff line number Diff line
@@ -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

+0 −4
Original line number Diff line number Diff line
@@ -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:

	/*