mm/codetag: swap tags when migrate pages

Current solution to adjust codetag references during page migration is
done in 3 steps:

1. sets the codetag reference of the old page as empty (not pointing
   to any codetag);

2. subtracts counters of the new page to compensate for its own
   allocation;

3. sets codetag reference of the new page to point to the codetag of
   the old page.

This does not work if CONFIG_MEM_ALLOC_PROFILING_DEBUG=n because
set_codetag_empty() becomes NOOP.  Instead, let's simply swap codetag
references so that the new page is referencing the old codetag and the old
page is referencing the new codetag.  This way accounting stays valid and
the logic makes more sense.

Link: https://lkml.kernel.org/r/20241129025213.34836-1-00107082@163.com
Fixes: e0a955b ("mm/codetag: add pgalloc_tag_copy()")
Signed-off-by: David Wang <00107082@163.com>
Closes: https://lore.kernel.org/lkml/20241124074318.399027-1-00107082@163.com/
Acked-by: Suren Baghdasaryan <surenb@google.com>
Suggested-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Yu Zhao <yuzhao@google.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/pgalloc_tag.h

@@ -231,7 +231,7 @@ static inline void pgalloc_tag_sub_pages(struct alloc_tag *tag, unsigned int nr)
 }
 
 void pgalloc_tag_split(struct folio *folio, int old_order, int new_order);
-void pgalloc_tag_copy(struct folio *new, struct folio *old);
+void pgalloc_tag_swap(struct folio *new, struct folio *old);
 
 void __init alloc_tag_sec_init(void);
 
@@ -245,7 +245,7 @@ static inline struct alloc_tag *pgalloc_tag_get(struct page *page) { return NULL
 static inline void pgalloc_tag_sub_pages(struct alloc_tag *tag, unsigned int nr) {}
 static inline void alloc_tag_sec_init(void) {}
 static inline void pgalloc_tag_split(struct folio *folio, int old_order, int new_order) {}
-static inline void pgalloc_tag_copy(struct folio *new, struct folio *old) {}
+static inline void pgalloc_tag_swap(struct folio *new, struct folio *old) {}
 
 #endif /* CONFIG_MEM_ALLOC_PROFILING */
 

lib/alloc_tag.c

@@ -189,26 +189,34 @@ void pgalloc_tag_split(struct folio *folio, int old_order, int new_order)
 	}
 }
 
-void pgalloc_tag_copy(struct folio *new, struct folio *old)
+void pgalloc_tag_swap(struct folio *new, struct folio *old)
 {
-	union pgtag_ref_handle handle;
-	union codetag_ref ref;
-	struct alloc_tag *tag;
+	union pgtag_ref_handle handle_old, handle_new;
+	union codetag_ref ref_old, ref_new;
+	struct alloc_tag *tag_old, *tag_new;
 
-	tag = pgalloc_tag_get(&old->page);
-	if (!tag)
+	tag_old = pgalloc_tag_get(&old->page);
+	if (!tag_old)
+		return;
+	tag_new = pgalloc_tag_get(&new->page);
+	if (!tag_new)
 		return;
 
-	if (!get_page_tag_ref(&new->page, &ref, &handle))
+	if (!get_page_tag_ref(&old->page, &ref_old, &handle_old))
 		return;
+	if (!get_page_tag_ref(&new->page, &ref_new, &handle_new)) {
+		put_page_tag_ref(handle_old);
+		return;
+	}
+
+	/* swap tags */
+	__alloc_tag_ref_set(&ref_old, tag_new);
+	update_page_tag_ref(handle_old, &ref_old);
+	__alloc_tag_ref_set(&ref_new, tag_old);
+	update_page_tag_ref(handle_new, &ref_new);
 
-	/* Clear the old ref to the original allocation tag. */
-	clear_page_tag_ref(&old->page);
-	/* Decrement the counters of the tag on get_new_folio. */
-	alloc_tag_sub(&ref, folio_size(new));
-	__alloc_tag_ref_set(&ref, tag);
-	update_page_tag_ref(handle, &ref);
-	put_page_tag_ref(handle);
+	put_page_tag_ref(handle_old);
+	put_page_tag_ref(handle_new);
 }
 
 static void shutdown_mem_profiling(bool remove_file)

mm/migrate.c

@@ -745,7 +745,7 @@ void folio_migrate_flags(struct folio *newfolio, struct folio *folio)
 		folio_set_readahead(newfolio);
 
 	folio_copy_owner(newfolio, folio);
-	pgalloc_tag_copy(newfolio, folio);
+	pgalloc_tag_swap(newfolio, folio);
 
 	mem_cgroup_migrate(folio, newfolio);
 }