[BUG] refill_stock -- VM_WARN_ON_ONCE(mem_cgroup_is_root()) from a write to the cgroup-v2 root's memory.reclaim

[Farhad Alemi]

Hello,

As part of the kernel research at ASU's SEFCOM
lab, we hit the crash below. Crash reports can be found here:

  https://github.com/farhad-alemi/public_bug_reports/tree/main/145-memcg-vm_warn-refill_stock-root-cgroup/

  mem_cgroup_is_root(memcg)
  WARNING: mm/memcontrol.c:2198 at refill_stock+0x644/0x940, CPU#1: repro/9490
  Call Trace:
   refill_stock+0x644/0x940
   try_charge_memcg+0x12d6/0x1570
   __obj_cgroup_charge+0x35/0xf0
   obj_cgroup_charge+0x1de/0x210
   obj_cgroup_charge_zswap+0x83/0x270
   zswap_store+0x1620/0x2000
   swap_writeout+0x94c/0x14c0
   shrink_folio_list+0x3388/0x52b0
   evict_folios+0x374d/0x48c0
   try_to_shrink_lruvec+0xe72/0x1350
   shrink_node+0x3320/0x3b90
   do_try_to_free_pages+0x6a2/0x1990
   try_to_free_mem_cgroup_pages+0x30d/0x830
   user_proactive_reclaim+0x504/0x840
   memory_reclaim+0x1f/0x30
   cgroup_file_write+0x331/0x8f0
   kernfs_fop_write_iter+0x3af/0x540

Our reproducer.c is available upon request.

Happy to test a patch if that would help.

Regards,