[PATCH v4 03/17] mm/memory: slightly simplify copy_present_pte()

David Hildenbrand
2022-04-28T08:38:30Z

Let's move the pinning check into the caller, to simplify return code
logic and prepare for further changes: relocating the
page_needs_cow_for_dma() into rmap handling code.

While at it, remove the unused pte parameter and simplify the comments a
bit.

No functional change intended.

Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
mm/memory.c | 53 ++++++++++++++++-------------------------------------
1 file changed, 16 insertions(+), 37 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 76e3af9639d9..ca0b256d1065 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -862,19 +862,11 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
}

/*
- * Copy a present and normal page if necessary.
+ * Copy a present and normal page.
*
- * NOTE! The usual case is that this doesn't need to do
- * anything, and can just return a positive value. That
- * will let the caller know that it can just increase
- * the page refcount and re-use the pte the traditional
- * way.
- *
- * But _if_ we need to copy it because it needs to be
- * pinned in the parent (and the child should get its own
- * copy rather than just a reference to the same page),
- * we'll do that here and return zero to let the caller
- * know we're done.
+ * NOTE! The usual case is that this isn't required;
+ * instead, the caller can just increase the page refcount
+ * and re-use the pte the traditional way.
*
* And if we need a pre-allocated page but don't yet have
* one, return a negative error to let the preallocation
@@ -884,25 +876,10 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
static inline int
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 page **prealloc, struct page *page)
{
struct page *new_page;
-
- /*
- * What we want to do is to check whether this page may
- * have been pinned by the parent process. If so,
- * instead of wrprotect the pte on both sides, we copy
- * the page immediately so that we'll always guarantee
- * the pinned page won't be randomly replaced in the
- * future.
- *
- * The page pinning checks are just "has this mm ever
- * seen pinning", along with the (inexact) check of
- * the page count. That might give false positives for
- * for pinning, but it will work correctly.
- */
- if (likely(!page_needs_cow_for_dma(src_vma, page)))
- return 1;
+ pte_t pte;

new_page = *prealloc;
if (!new_page)
@@ -944,14 +921,16 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
struct page *page;

page = vm_normal_page(src_vma, addr, pte);
- if (page) {
- int retval;
-
- retval = copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
- addr, rss, prealloc, pte, page);
- if (retval <= 0)
- return retval;
-
+ if (page && unlikely(page_needs_cow_for_dma(src_vma, page))) {
+ /*
+ * If this page may have been pinned by the parent process,
+ * copy the page immediately for the child so that we'll always
+ * guarantee the pinned page won't be randomly replaced in the
+ * future.
+ */
+ return copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
+ addr, rss, prealloc, page);
+ } else if (page) {
get_page(page);
page_dup_rmap(page, false);
rss[mm_counter(page)]++;
--
2.35.1