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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: David Hildenbrand <david@redhat.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
David Hildenbrand 2022-11-16 11:26:47 +01:00 committed by Andrew Morton
parent aea06577a9
commit 8d6a0ac09a
2 changed files with 4 additions and 8 deletions
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8
include/linux/mm_types.h
View File

@ -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,

4
mm/memory.c
View File

@ -3431,10 +3431,6 @@ reuse:
}
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:
/*