mm: multi-gen LRU: fix NULL lruvec->pgdat deref with memcg disabled on cmdline

When CONFIG_MEMCG=y, lruvec_pgdat() returns lruvec->pgdat, which for the
per-node pgdat->__lruvec is left NULL by lruvec_init() and only filled in
lazily by mem_cgroup_lruvec(). Normally that happens early, e.g. on the
first folio_add_lru().

With cgroup_disable=memory, lru_gen_shrink_node() passes &pgdat->__lruvec
directly to shrink_one(). If direct reclaim runs before anything has
called mem_cgroup_lruvec() for the node, which can happen during early
boot under memory pressure (e.g. a kdump kernel), get_swappiness() ends
up dereferencing a NULL pgdat:

  Unable to handle kernel NULL pointer dereference at virtual address 0000000000005040
  pc : get_swappiness+0x68/0xb0
  lr : try_to_shrink_lruvec+0x54/0x2c0
  Call trace:
   get_swappiness+0x68/0xb0 (P)
   try_to_shrink_lruvec+0x54/0x2c0
   shrink_one+0x8c/0x240
   shrink_node+0xc6c/0xe08
   do_try_to_free_pages+0xdc/0x570
   try_to_free_pages+0x124/0x278
   __alloc_frozen_pages_noprof+0x6e8/0xf78
   alloc_pages_mpol+0xc4/0x1c0
   alloc_frozen_pages_noprof+0x54/0xe8
   alloc_pages_noprof+0x1c/0x40
   atomic_pool_expand+0x98/0x300
   __dma_atomic_pool_init+0x58/0xd8
   dma_atomic_pool_init+0x154/0x170
   do_one_initcall+0x64/0x350
   kernel_init_freeable+0x214/0x430
   kernel_init+0x2c/0x208
   ret_from_fork+0x10/0x20

The classic LRU path and MGLRU before per-node lru_gen_folio lists
obtained the lruvec through mem_cgroup_lruvec(), which sets
lruvec->pgdat.

The above was hit on an arm64 kdump kernel (6.18-based) booted with
cgroup_disable=memory nr_cpus=4. It also reproduces on mainline on an
arm64 VM with CONFIG_ARM64_64K_PAGES=y, CONFIG_MEMCG=y,
CONFIG_LRU_GEN_ENABLED=y and:

  mem=160M nr_cpus=4 coherent_pool=512M cgroup_disable=memory

These options only serve to force direct reclaim early in boot; with
160M the boot eventually ends in OOM regardless. On mainline the first
LRU reclaim happens while unpacking the initramfs, and faults the same
way:

  Unable to handle kernel NULL pointer dereference at virtual address 0000000000005040
  pc : get_swappiness+0xd4/0x150
  Call trace:
   get_swappiness+0xd4/0x150 (P)
   try_to_shrink_lruvec+0x44/0x2b8
   shrink_one+0x80/0x1a0
   shrink_node+0xd98/0xea8
   do_try_to_free_pages+0xf4/0x4b0
   try_to_free_pages+0x110/0x360
   __alloc_frozen_pages_noprof+0x8b0/0x1718
   ...
   __filemap_get_folio_mpol+0x2ac/0x548
   simple_write_begin+0x44/0xf8
   page_symlink+0x178/0x198
   ...
   do_populate_rootfs+0x13c/0x200

Link: https://lore.kernel.org/20261017050928.128582-1-sidchintamaneni@gmail.com
Fixes: e4dde56cd2 ("mm: multi-gen LRU: per-node lru_gen_folio lists")
Signed-off-by: Siddharth Chintamaneni <sidchintamaneni@gmail.com>
Signed-off-by: Hardik Garg <hargar@linux.microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Assisted-by: LLM
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Yu Zhao <yuzhao@google.com>
Cc: <apais@microsoft.com>
Cc: <rlmenge@gmail.com>
Cc: <stable@vger.kernel.org> # 6.3+

mm/vmscan.c

@@ -5454,7 +5454,7 @@ static void lru_gen_shrink_node(struct pglist_data *pgdat, struct scan_control *
 		sc->nr_reclaimed = 0;
 
 	if (mem_cgroup_disabled())
-		shrink_one(&pgdat->__lruvec, sc);
+		shrink_one(mem_cgroup_lruvec(NULL, pgdat), sc);
 	else
 		shrink_many(pgdat, sc);