[PATCH] mm: vmscan: always allow writeback during memcg reclaim

[Deepanshu Kartikey <kartikey406@gmail.com>]

When laptop_mode is enabled, may_writepage is set to 0 in
try_to_free_mem_cgroup_pages(). This triggers a warning in MGLRU's
lru_gen_shrink_lruvec():

    VM_WARN_ON_ONCE(!sc->may_writepage || !sc->may_unmap);

The warning occurs because MGLRU expects full reclaim capabilities to
function correctly. The call path is:

    mem_cgroup_resize_max()
      try_to_free_mem_cgroup_pages()
        do_try_to_free_pages()
          shrink_node()
            shrink_lruvec()
              lru_gen_shrink_lruvec()  <-- WARNING

Unlike kswapd or direct reclaim where laptop_mode's disk-saving behavior
is a reasonable optimization, memcg limit enforcement is a hard
requirement - memory MUST be freed when a cgroup exceeds its limit.
The may_unmap field is already set unconditionally to 1 in this path,
acknowledging that memcg reclaim needs full capabilities.

Set may_writepage unconditionally to 1 for memcg reclaim to ensure
MGLRU works correctly and memory limits are properly enforced.

Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks")
Reported-by: syzbot+90fcab4d88cffed6d0d8@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=90fcab4d88cffed6d0d8
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>

---
Note: Only compile-tested. No reproducer available from syzbot.
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 900c74b6aa62..5e1c99d9cbd7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -6669,7 +6669,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
 		.reclaim_idx = MAX_NR_ZONES - 1,
 		.target_mem_cgroup = memcg,
 		.priority = DEF_PRIORITY,
-		.may_writepage = !laptop_mode,
+		.may_writepage = 1,
 		.may_unmap = 1,
 		.may_swap = !!(reclaim_options & MEMCG_RECLAIM_MAY_SWAP),
 		.proactive = !!(reclaim_options & MEMCG_RECLAIM_PROACTIVE),
-- 
2.43.0