[PATCH] mm, madvise: fix potential workingset node list_lru leaks

[Kairui Song]

From: Kairui Song <kasong@xxxxxxxxxxx>

Since commit 5abc1e37afa0 ("mm: list_lru: allocate list_lru_one
only when needed"), all list_lru users need to allocate the items using
the new infrastructure that provides list_lru info for slab allocation,
ensuring that the corresponding memcg list_lru is allocated before use.

For workingset shadow nodes (which are xa_node), users are converted to
use the new infrastructure by commit 9bbdc0f32409 ("xarray: use
kmem_cache_alloc_lru to allocate xa_node"). The xas->xa_lru will be
set correctly for filemap users. However, there is a missing case:
xa_node allocations caused by madvise(..., MADV_COLLAPSE).

madvise(..., MADV_COLLAPSE) will also read in the absent parts of file
map, and there will be xa_nodes allocated for the caller's memcg
(assuming it's not rootcg). However, these allocations won't trigger
memcg list_lru allocation because the proper xas info was not set.

If nothing else has allocated other xa_nodes for that memcg to trigger
list_lru creation, and memory pressure starts to evict file pages,
workingset_update_node will try to add these xa_nodes to their
corresponding memcg list_lru, and it does not exist (NULL). So they
will be added to rootcg's list_lru instead.

This shouldn't be a significant issue in practice, but it is indeed
unexpected behavior, and these xa_nodes will not be reclaimed
effectively. And may lead to incorrect counting of the
list_lru->nr_items counter.

This problem wasn't exposed until recent commit 28e98022b31ef
("mm/list_lru: simplify reparenting and initial allocation") added a
sanity check: only dying memcg could have a NULL list_lru when
list_lru_{add,del} is called. This problem triggered this WARNING.

So make madvise(..., MADV_COLLAPSE) also call xas_set_lru() to pass
the list_lru which we may want to insert xa_node into later. And move
mapping_set_update to mm/internal.h, and turn into a macro to avoid
including extra headers in mm/internal.h.

Fixes: 9bbdc0f32409 ("xarray: use kmem_cache_alloc_lru to allocate xa_node")
Reported-by: syzbot+38a0cbd267eff2d286ff@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/lkml/675d01e9.050a0220.37aaf.00be.GAE@xxxxxxxxxx/
Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
 mm/filemap.c    | 9 ---------
 mm/internal.h   | 6 ++++++
 mm/khugepaged.c | 3 +++
 3 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index f61cf51c2238..33b60d448fca 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -124,15 +124,6 @@
  *    ->private_lock		(zap_pte_range->block_dirty_folio)
  */
 
-static void mapping_set_update(struct xa_state *xas,
-		struct address_space *mapping)
-{
-	if (dax_mapping(mapping) || shmem_mapping(mapping))
-		return;
-	xas_set_update(xas, workingset_update_node);
-	xas_set_lru(xas, &shadow_nodes);
-}
-
 static void page_cache_delete(struct address_space *mapping,
 				   struct folio *folio, void *shadow)
 {
diff --git a/mm/internal.h b/mm/internal.h
index cb8d8e8e3ffa..4e7a3a93d0a2 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1510,6 +1510,12 @@ static inline void shrinker_debugfs_remove(struct dentry *debugfs_entry,
 /* Only track the nodes of mappings with shadow entries */
 void workingset_update_node(struct xa_node *node);
 extern struct list_lru shadow_nodes;
+#define mapping_set_update(xas, mapping) do {			\
+	if (!dax_mapping(mapping) && !shmem_mapping(mapping)) {	\
+		xas_set_update(xas, workingset_update_node);	\
+		xas_set_lru(xas, &shadow_nodes);		\
+	}							\
+} while (0)
 
 /* mremap.c */
 unsigned long move_page_tables(struct vm_area_struct *vma,
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 6f8d46d107b4..653dbb1ff05c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -19,6 +19,7 @@
 #include <linux/rcupdate_wait.h>
 #include <linux/swapops.h>
 #include <linux/shmem_fs.h>
+#include <linux/dax.h>
 #include <linux/ksm.h>
 
 #include <asm/tlb.h>
@@ -1837,6 +1838,8 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	if (result != SCAN_SUCCEED)
 		goto out;
 
+	mapping_set_update(&xas, mapping);
+
 	__folio_set_locked(new_folio);
 	if (is_shmem)
 		__folio_set_swapbacked(new_folio);
-- 
2.47.1