mm: introduce page_needs_cow_for_dma() for deciding whether cow

We've got quite a few places (pte, pmd, pud) that explicitly checked
against whether we should break the cow right now during fork().  It's
easier to provide a helper, especially before we work the same thing on
hugetlbfs.

Since we'll reference is_cow_mapping() in mm.h, move it there too.
Actually it suites mm.h more since internal.h is mm/ only, but mm.h is
exported to the whole kernel.  With that we should expect another patch to
use is_cow_mapping() whenever we can across the kernel since we do use it
quite a lot but it's always done with raw code against VM_* flags.

Link: https://lkml.kernel.org/r/20210217233547.93892-4-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Gal Pressman <galpress@amazon.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Kirill Shutemov <kirill@shutemov.name>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Wei Zhang <wzam@amazon.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 7 deletions

diff --git a/mm/memory.c b/mm/memory.c
index c8e357627318..523230005db1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -809,12 +809,8 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
 		  pte_t *dst_pte, pte_t *src_pte, unsigned long addr, int *rss,
 		  struct page **prealloc, pte_t pte, struct page *page)
 {
-	struct mm_struct *src_mm = src_vma->vm_mm;
 	struct page *new_page;
 
-	if (!is_cow_mapping(src_vma->vm_flags))
-		return 1;
-
 	/*
 	 * What we want to do is to check whether this page may
 	 * have been pinned by the parent process.  If so,
@@ -828,9 +824,7 @@ copy_present_page(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma
 	 * the page count. That might give false positives for
 	 * for pinning, but it will work correctly.
 	 */
-	if (likely(!atomic_read(&src_mm->has_pinned)))
-		return 1;
-	if (likely(!page_maybe_dma_pinned(page)))
+	if (likely(!page_needs_cow_for_dma(src_vma, page)))
 		return 1;
 
 	new_page = *prealloc;