[PATCH 1/7] mm/swap, workingset: make anon workingset nodes memcg aware

[Kairui Song <ryncsn@gmail.com>]

From: Kairui Song <kasong@tencent.com>

Currently, the (shadow) nodes of the swap cache are not accounted to
their corresponding memory cgroup, instead, they are all accounted
to the root cgroup. This leads to inaccurate accounting and
ineffective reclaiming.

This issue is similar to commit 7b785645e8f1 ("mm: fix page cache
convergence regression"), where page cache shadow nodes were incorrectly
accounted. That was due to the accidental dropping of the accounting
flag during the XArray conversion in commit a28334862993
("page cache: Finish XArray conversion").

However, this fix has a different cause. Swap cache shadow nodes were
never accounted even before the XArray conversion, since they did not
exist until commit 3852f6768ede ("mm/swapcache: support to handle the
shadow entries"), which was years after the XArray conversion. Without
shadow nodes, swap cache nodes can only use a very small amount of memory
and so reclaiming is not very important.

But now with shadow nodes, if a cgroup swaps out a large amount of
memory, it could take up a lot of memory.

This can be easily fixed by adding proper flags and LRU setters.

Signed-off-by: Kairui Song <kasong@tencent.com>
---
 mm/swap_state.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 642c30d8376c..68afaaf1c09b 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -95,6 +95,7 @@ int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
 	void *old;
 
 	xas_set_update(&xas, workingset_update_node);
+	xas_set_lru(&xas, &shadow_nodes);
 
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
 	VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio);
@@ -714,7 +715,7 @@ int init_swap_address_space(unsigned int type, unsigned long nr_pages)
 		return -ENOMEM;
 	for (i = 0; i < nr; i++) {
 		space = spaces + i;
-		xa_init_flags(&space->i_pages, XA_FLAGS_LOCK_IRQ);
+		xa_init_flags(&space->i_pages, XA_FLAGS_LOCK_IRQ | XA_FLAGS_ACCOUNT);
 		atomic_set(&space->i_mmap_writable, 0);
 		space->a_ops = &swap_aops;
 		/* swap cache doesn't use writeback related tags */
-- 
2.45.2