[101/114] mm: slightly clarify KSM logic in do_swap_page()

Commit Message

Andrew Morton March 25, 2022, 1:13 a.m. UTC

From: David Hildenbrand <[e-mail]>
Subject: mm: slightly clarify KSM logic in do_swap_page()

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.

[[e-mail]: fix comment, per David]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: David Hildenbrand <[e-mail]>
Acked-by: Vlastimil Babka <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: David Rientjes <[e-mail]>
Cc: Don Dutile <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: John Hubbard <[e-mail]>
Cc: Kirill A. Shutemov <[e-mail]>
Cc: Liang Zhang <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Mike Kravetz <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Nadav Amit <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Shakeel Butt <[e-mail]>
Cc: Yang Shi <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/memory.c |   36 ++++++++++++++++++++++--------------
 1 file changed, 22 insertions(+), 14 deletions(-)

Patch

--- a/mm/memory.c~mm-slightly-clarify-ksm-logic-in-do_swap_page
+++ a/mm/memory.c
@@ -3607,21 +3607,29 @@  vm_fault_t do_swap_page(struct vm_fault
 		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;
+	if (swapcache) {
+		/*
+		 * Make sure try_to_free_swap 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;
 
-	page = ksm_might_need_to_copy(page, vma, vmf->address);
-	if (unlikely(!page)) {
-		ret = VM_FAULT_OOM;
-		page = swapcache;
-		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);