mm/page_owner: record the timestamp of all pages during free

Collect the time when each allocation is freed, to help with memory
analysis with kdump/ramdump.  Add the timestamp also in the page_owner
debugfs file and print it in dump_page().

Having another timestamp when we free the page helps for debugging page
migration issues.  For example both alloc and free timestamps being the
same can gave hints that there is an issue with migrating memory, as
opposed to a page just being dropped during migration.

Link: https://lkml.kernel.org/r/20210203175905.12267-1-georgi.djakov@linaro.org
Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/vm/page_owner.rst

@@ -47,7 +47,7 @@ size change due to this facility.
 
    text    data     bss     dec     hex filename
    48800   2445     644   51889    cab1 mm/page_alloc.o
-   6574     108      29    6711    1a37 mm/page_owner.o
+   6662     108      29    6799    1a8f mm/page_owner.o
    1025       8       8    1041     411 mm/page_ext.o
 
 Although, roughly, 8 KB code is added in total, page_alloc.o increase by

mm/page_owner.c

@@ -27,6 +27,7 @@ struct page_owner {
 	depot_stack_handle_t handle;
 	depot_stack_handle_t free_handle;
 	u64 ts_nsec;
+	u64 free_ts_nsec;
 	pid_t pid;
 };
 
@@ -148,6 +149,7 @@ void __reset_page_owner(struct page *page, unsigned int order)
 	struct page_ext *page_ext;
 	depot_stack_handle_t handle = 0;
 	struct page_owner *page_owner;
+	u64 free_ts_nsec = local_clock();
 
 	handle = save_stack(GFP_NOWAIT | __GFP_NOWARN);
 
@@ -158,6 +160,7 @@ void __reset_page_owner(struct page *page, unsigned int order)
 		__clear_bit(PAGE_EXT_OWNER_ALLOCATED, &page_ext->flags);
 		page_owner = get_page_owner(page_ext);
 		page_owner->free_handle = handle;
+		page_owner->free_ts_nsec = free_ts_nsec;
 		page_ext = page_ext_next(page_ext);
 	}
 }
@@ -243,6 +246,7 @@ void __copy_page_owner(struct page *oldpage, struct page *newpage)
 	new_page_owner->handle = old_page_owner->handle;
 	new_page_owner->pid = old_page_owner->pid;
 	new_page_owner->ts_nsec = old_page_owner->ts_nsec;
+	new_page_owner->free_ts_nsec = old_page_owner->ts_nsec;
 
 	/*
 	 * We don't clear the bit on the oldpage as it's going to be freed
@@ -356,10 +360,10 @@ print_page_owner(char __user *buf, size_t count, unsigned long pfn,
 		return -ENOMEM;
 
 	ret = snprintf(kbuf, count,
-			"Page allocated via order %u, mask %#x(%pGg), pid %d, ts %llu ns\n",
+			"Page allocated via order %u, mask %#x(%pGg), pid %d, ts %llu ns, free_ts %llu ns\n",
 			page_owner->order, page_owner->gfp_mask,
 			&page_owner->gfp_mask, page_owner->pid,
-			page_owner->ts_nsec);
+			page_owner->ts_nsec, page_owner->free_ts_nsec);
 
 	if (ret >= count)
 		goto err;
@@ -435,9 +439,9 @@ void __dump_page_owner(struct page *page)
 	else
 		pr_alert("page_owner tracks the page as freed\n");
 
-	pr_alert("page last allocated via order %u, migratetype %s, gfp_mask %#x(%pGg), pid %d, ts %llu\n",
+	pr_alert("page last allocated via order %u, migratetype %s, gfp_mask %#x(%pGg), pid %d, ts %llu, free_ts %llu\n",
 		 page_owner->order, migratetype_names[mt], gfp_mask, &gfp_mask,
-		 page_owner->pid, page_owner->ts_nsec);
+		 page_owner->pid, page_owner->ts_nsec, page_owner->free_ts_nsec);
 
 	handle = READ_ONCE(page_owner->handle);
 	if (!handle) {