[PATCH v3 3/9] mm: slightly clarify KSM logic in do_swap_page()

[David Hildenbrand <david@redhat.com>]

Let's make it clearer that KSM might only have to copy a page
in case we have a page in the swapcache, not if we allocated a fresh
page and bypassed the swapcache. While at it, add a comment why this is
usually necessary and merge the two swapcache conditions.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/memory.c | 38 +++++++++++++++++++++++---------------
 1 file changed, 23 insertions(+), 15 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 923165b4c27e..3c91294cca98 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3615,21 +3615,29 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 		goto out_release;
 	}
 
-	/*
-	 * Make sure try_to_free_swap or reuse_swap_page or swapoff did not
-	 * release the swapcache from under us.  The page pin, and pte_same
-	 * test below, are not enough to exclude that.  Even if it is still
-	 * swapcache, we need to check that the page's swap has not changed.
-	 */
-	if (unlikely((!PageSwapCache(page) ||
-			page_private(page) != entry.val)) && swapcache)
-		goto out_page;
-
-	page = ksm_might_need_to_copy(page, vma, vmf->address);
-	if (unlikely(!page)) {
-		ret = VM_FAULT_OOM;
-		page = swapcache;
-		goto out_page;
+	if (swapcache) {
+		/*
+		 * Make sure try_to_free_swap or reuse_swap_page or swapoff did
+		 * not release the swapcache from under us.  The page pin, and
+		 * pte_same test below, are not enough to exclude that.  Even if
+		 * it is still swapcache, we need to check that the page's swap
+		 * has not changed.
+		 */
+		if (unlikely(!PageSwapCache(page) ||
+			     page_private(page) != entry.val))
+			goto out_page;
+
+		/*
+		 * KSM sometimes has to copy on read faults, for example, if
+		 * page->index of !PageKSM() pages would be nonlinear inside the
+		 * anon VMA -- PageKSM() is lost on actual swapout.
+		 */
+		page = ksm_might_need_to_copy(page, vma, vmf->address);
+		if (unlikely(!page)) {
+			ret = VM_FAULT_OOM;
+			page = swapcache;
+			goto out_page;
+		}
 	}
 
 	cgroup_throttle_swaprate(page, GFP_KERNEL);
-- 
2.34.1