[PATCH v2 1/3] mm/ksm: refactor out try_to_merge_with_zero_page()

Chengming Zhou
2024-06-21T07:55:11Z

In preparation for later changes, refactor out a new function called
try_to_merge_with_zero_page(), which tries to merge with zero page.

Signed-off-by: Chengming Zhou <chengming.zhou@linux.dev>
---
mm/ksm.c | 70 ++++++++++++++++++++++++++++++++++++----------------------------
1 file changed, 40 insertions(+), 30 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 34c4820e0d3d..1427abd18627 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1531,6 +1531,44 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
return err;
}

+/*
+ * This function returns 0 if the pages were merged or if they are
+ * no longer merging candidates (e.g., VMA stale), -EFAULT otherwise.
+ */
+static int try_to_merge_with_zero_page(struct ksm_rmap_item *rmap_item,
+ struct page *page)
+{
+ struct mm_struct *mm = rmap_item->mm;
+ int err = -EFAULT;
+
+ /*
+ * Same checksum as an empty page. We attempt to merge it with the
+ * appropriate zero page if the user enabled this via sysfs.
+ */
+ if (ksm_use_zero_pages && (rmap_item->oldchecksum == zero_checksum)) {
+ struct vm_area_struct *vma;
+
+ mmap_read_lock(mm);
+ vma = find_mergeable_vma(mm, rmap_item->address);
+ if (vma) {
+ err = try_to_merge_one_page(vma, page,
+ ZERO_PAGE(rmap_item->address));
+ trace_ksm_merge_one_page(
+ page_to_pfn(ZERO_PAGE(rmap_item->address)),
+ rmap_item, mm, err);
+ } else {
+ /*
+ * If the vma is out of date, we do not need to
+ * continue.
+ */
+ err = 0;
+ }
+ mmap_read_unlock(mm);
+ }
+
+ return err;
+}
+
/*
* try_to_merge_with_ksm_page - like try_to_merge_two_pages,
* but no new kernel page is allocated: kpage must already be a ksm page.
@@ -2306,7 +2344,6 @@ static void stable_tree_append(struct ksm_rmap_item *rmap_item,
*/
static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_item)
{
- struct mm_struct *mm = rmap_item->mm;
struct ksm_rmap_item *tree_rmap_item;
struct page *tree_page = NULL;
struct ksm_stable_node *stable_node;
@@ -2375,36 +2412,9 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite
return;
}

- /*
- * Same checksum as an empty page. We attempt to merge it with the
- * appropriate zero page if the user enabled this via sysfs.
- */
- if (ksm_use_zero_pages && (checksum == zero_checksum)) {
- struct vm_area_struct *vma;
+ if (!try_to_merge_with_zero_page(rmap_item, page))
+ return;

- mmap_read_lock(mm);
- vma = find_mergeable_vma(mm, rmap_item->address);
- if (vma) {
- err = try_to_merge_one_page(vma, page,
- ZERO_PAGE(rmap_item->address));
- trace_ksm_merge_one_page(
- page_to_pfn(ZERO_PAGE(rmap_item->address)),
- rmap_item, mm, err);
- } else {
- /*
- * If the vma is out of date, we do not need to
- * continue.
- */
- err = 0;
- }
- mmap_read_unlock(mm);
- /*
- * In case of failure, the page was not really empty, so we
- * need to continue. Otherwise we're done.
- */
- if (!err)
- return;
- }
tree_rmap_item =
unstable_tree_search_insert(rmap_item, page, &tree_page);
if (tree_rmap_item) {

--
2.45.2