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.
Signed-off-by: David Hildenbrand david@redhat.com --- include/linux/mm_types.h | 8 ++++---- mm/memory.c | 4 ---- 2 files changed, 4 insertions(+), 8 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 834022721bc6..3f9fa01a3e24 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -965,9 +965,9 @@ 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. * @@ -992,7 +992,7 @@ 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, diff --git a/mm/memory.c b/mm/memory.c index d2f9673755be..73ed83def548 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3431,10 +3431,6 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf) } wp_page_reuse(vmf); return 0; - } else if (unshare) { - /* No anonymous page -> nothing to do. */ - pte_unmap_unlock(vmf->pte, vmf->ptl); - return 0; } copy: /*