On 12/14/21 15:43, Johannes Weiner wrote:

On Wed, Dec 01, 2021 at 07:15:00PM +0100, Vlastimil Babka wrote:

quoted

page->memcg_data is used with MEMCG_DATA_OBJCGS flag only for slab pages
so convert all the related infrastructure to struct slab.

To avoid include cycles, move the inline definitions of slab_objcgs() and
slab_objcgs_check() from memcontrol.h to mm/slab.h.

This is not just mechanistic changing of types and names. Now in
mem_cgroup_from_obj() we use PageSlab flag to decide if we interpret the page
as slab, instead of relying on MEMCG_DATA_OBJCGS bit checked in
page_objcgs_check() (now slab_objcgs_check()). Similarly in
memcg_slab_free_hook() where we can encounter kmalloc_large() pages (here the
PageSlab flag check is implied by virt_to_slab()).

Yup, this is great.

quoted

@@ -2865,24 +2865,31 @@ int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s,
  */
 struct mem_cgroup *mem_cgroup_from_obj(void *p)
 {
-	struct page *page;
+	struct folio *folio;
 
 	if (mem_cgroup_disabled())
 		return NULL;
 
-	page = virt_to_head_page(p);
+	folio = virt_to_folio(p);
 
 	/*
 	 * Slab objects are accounted individually, not per-page.
 	 * Memcg membership data for each individual object is saved in
 	 * the page->obj_cgroups.
 	 */
-	if (page_objcgs_check(page)) {
+	if (folio_test_slab(folio)) {
+		struct obj_cgroup **objcgs;
 		struct obj_cgroup *objcg;
+		struct slab *slab;
 		unsigned int off;
 
-		off = obj_to_index(page->slab_cache, page_slab(page), p);
-		objcg = page_objcgs(page)[off];
+		slab = folio_slab(folio);
+		objcgs = slab_objcgs_check(slab);

AFAICS the change to the _check() variant was accidental.

folio_test_slab() makes sure it's a slab page, so the legit options
for memcg_data are NULL or |MEMCG_DATA_OBJCGS; using slab_objcgs()
here would include the proper asserts, like page_objcgs() used to.

Well spotted. On closer look, it's actually the same in
memcg_slab_free_hook() where I also added a folio_test_slab() as part of
virt_to_slab(). In fact it means page_objcgs_check() can be just deleted
instead of replaced by slab_objcgs_check(). Thanks!