[v3,03/16] mm/memory: slightly simplify copy_present_pte()

Commit Message

David Hildenbrand March 29, 2022, 4:04 p.m. UTC

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.

Signed-off-by: David Hildenbrand <[e-mail]>
---
 mm/memory.c | 53 ++++++++++++++++-------------------------------------
 1 file changed, 16 insertions(+), 37 deletions(-)

Comments

Vlastimil Babka April 11, 2022, 4:38 p.m. UTC | #1

On 3/29/22 18:04, David Hildenbrand wrote:
> 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.
> 
> Signed-off-by: David Hildenbrand <[e-mail]>

Yeah, much better.

Acked-by: Vlastimil Babka <[e-mail]>

Patch

diff --git a/mm/memory.c b/mm/memory.c
index be44d0b36b18..e3038cb6f212 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)]++;