HWPOISON: comment dirty swapcache pages

AK: Improve comment
Signed-off-by: NWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: NAndi Kleen <ak@linux.intel.com>

mm/memory.c

@@ -2553,6 +2553,10 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		ret = VM_FAULT_MAJOR;
 		count_vm_event(PGMAJFAULT);
 	} else if (PageHWPoison(page)) {
+		/*
+		 * hwpoisoned dirty swapcache pages are kept for killing
+		 * owner processes (which may be unknown at hwpoison time)
+		 */
 		ret = VM_FAULT_HWPOISON;
 		delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
 		goto out_release;