Re: [PATCH v3 10/18] mm: Allow non-hugetlb large folios to be batch processed

[Matthew Wilcox <willy@infradead.org>]

On Sun, Mar 10, 2024 at 08:46:58PM +0000, Matthew Wilcox wrote:
> On Sun, Mar 10, 2024 at 07:59:46PM +0000, Ryan Roberts wrote:
> > I've now been able to repro this without any of my code on top - just mm-unstable and your fix for the the memcg uncharging ordering issue. So we have separate, more difficultt to repro bug. I've discovered CONFIG_DEBUG_LIST so enabled that. I'll try to bisect in the morning, but I suspect it will be slow going.
> > 
> > [  390.317982] ------------[ cut here ]------------
> > [  390.318646] list_del corruption. prev->next should be fffffc00152a9090, but was fffffc002798a490. (prev=fffffc002798a490)
> 
> Interesting.  So prev->next is pointing to prev, ie prev is an empty
> list, but it should be pointing to this entry ... this is feeling like
> another missing lock.

Let's check that we're not inverting the order of memcg_uncharge and
removing a folio from the deferred list (build tested only, but only
one line of this will be new to you):

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index bb57b3d0c8cd..61fd1a4b424d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -792,8 +792,6 @@ void folio_prep_large_rmappable(struct folio *folio)
 {
 	if (!folio || !folio_test_large(folio))
 		return;
-	if (folio_order(folio) > 1)
-		INIT_LIST_HEAD(&folio->_deferred_list);
 	folio_set_large_rmappable(folio);
 }
 
diff --git a/mm/internal.h b/mm/internal.h
index 79d0848c10a5..690c68c18c23 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -525,6 +525,8 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
 	atomic_set(&folio->_entire_mapcount, -1);
 	atomic_set(&folio->_nr_pages_mapped, 0);
 	atomic_set(&folio->_pincount, 0);
+	if (order > 1)
+		INIT_LIST_HEAD(&folio->_deferred_list);
 }
 
 static inline void prep_compound_tail(struct page *head, int tail_idx)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 138bcfa18234..e2334c4ee550 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7483,6 +7483,8 @@ static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
 	struct obj_cgroup *objcg;
 
 	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
+	VM_BUG_ON_FOLIO(folio_order(folio) > 1 &&
+			!list_empty(&folio->_deferred_list), folio);
 
 	/*
 	 * Nobody should be changing or seriously looking at
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index bdff5c0a7c76..1c1925b92934 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1006,10 +1006,11 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
 		}
 		break;
 	case 2:
-		/*
-		 * the second tail page: ->mapping is
-		 * deferred_list.next -- ignore value.
-		 */
+		/* the second tail page: deferred_list overlaps ->mapping */
+		if (unlikely(!list_empty(&folio->_deferred_list))) {
+			bad_page(page, "on deferred list");
+			goto out;
+		}
 		break;
 	default:
 		if (page->mapping != TAIL_MAPPING) {