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@xxxxxxxxxx>
---
mm/memory.c | 38 +++++++++++++++++++++++---------------
1 file changed, 23 insertions(+), 15 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index 61d67ceef734..ab3153252cfe 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3617,21 +3617,29 @@ 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
