MGLRU premature memcg OOM on slow writes

[Chris Down <chris@chrisdown.name>]

Hi Yu,

When running with MGLRU I'm encountering premature OOMs when transferring files 
to a slow disk.

On non-MGLRU setups, writeback flushers are awakened and get to work. But on 
MGLRU, one can see OOM killer outputs like the following when doing an rsync 
with a memory.max of 32M:

---

     % systemd-run --user -t -p MemoryMax=32M -- rsync -rv ... /mnt/usb
     Running as unit: run-u640.service
     Press ^] three times within 1s to disconnect TTY.
     sending incremental file list
     ...
     rsync error: received SIGINT, SIGTERM, or SIGHUP (code 20) at rsync.c(713) [generator=3.2.7]

---

     [41368.535735] Memory cgroup out of memory: Killed process 128824 (rsync) total-vm:14008kB, anon-rss:256kB, file-rss:5504kB, shmem-rss:0kB, UID:1000 pgtables:64kB oom_score_adj:200
     [41369.847965] rsync invoked oom-killer: gfp_mask=0x408d40(GFP_NOFS|__GFP_NOFAIL|__GFP_ZERO|__GFP_ACCOUNT), order=0, oom_score_adj=200
     [41369.847972] CPU: 1 PID: 128826 Comm: rsync Tainted: G S         OE      6.7.4-arch1-1 #1 20d30c48b78a04be2046f4b305b40455f0b5b38b
     [41369.847975] Hardware name: LENOVO 20WNS23A0G/20WNS23A0G, BIOS N35ET53W (1.53 ) 03/22/2023
     [41369.847977] Call Trace:
     [41369.847978]  <TASK>
     [41369.847980]  dump_stack_lvl+0x47/0x60
     [41369.847985]  dump_header+0x45/0x1b0
     [41369.847988]  oom_kill_process+0xfa/0x200
     [41369.847990]  out_of_memory+0x244/0x590
     [41369.847992]  mem_cgroup_out_of_memory+0x134/0x150
     [41369.847995]  try_charge_memcg+0x76d/0x870
     [41369.847998]  ? try_charge_memcg+0xcd/0x870
     [41369.848000]  obj_cgroup_charge+0xb8/0x1b0
     [41369.848002]  kmem_cache_alloc+0xaa/0x310
     [41369.848005]  ? alloc_buffer_head+0x1e/0x80
     [41369.848007]  alloc_buffer_head+0x1e/0x80
     [41369.848009]  folio_alloc_buffers+0xab/0x180
     [41369.848012]  ? __pfx_fat_get_block+0x10/0x10 [fat 0a109de409393851f8a884f020fb5682aab8dcd1]
     [41369.848021]  create_empty_buffers+0x1d/0xb0
     [41369.848023]  __block_write_begin_int+0x524/0x600
     [41369.848026]  ? __pfx_fat_get_block+0x10/0x10 [fat 0a109de409393851f8a884f020fb5682aab8dcd1]
     [41369.848031]  ? __filemap_get_folio+0x168/0x2e0
     [41369.848033]  ? __pfx_fat_get_block+0x10/0x10 [fat 0a109de409393851f8a884f020fb5682aab8dcd1]
     [41369.848038]  block_write_begin+0x52/0x120
     [41369.848040]  fat_write_begin+0x34/0x80 [fat 0a109de409393851f8a884f020fb5682aab8dcd1]
     [41369.848046]  ? __pfx_fat_get_block+0x10/0x10 [fat 0a109de409393851f8a884f020fb5682aab8dcd1]
     [41369.848051]  generic_perform_write+0xd6/0x240
     [41369.848054]  generic_file_write_iter+0x65/0xd0
     [41369.848056]  vfs_write+0x23a/0x400
     [41369.848060]  ksys_write+0x6f/0xf0
     [41369.848063]  do_syscall_64+0x61/0xe0
     [41369.848065]  ? do_user_addr_fault+0x304/0x670
     [41369.848069]  ? exc_page_fault+0x7f/0x180
     [41369.848071]  entry_SYSCALL_64_after_hwframe+0x6e/0x76
     [41369.848074] RIP: 0033:0x7965df71a184
     [41369.848116] Code: c7 00 16 00 00 00 b8 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 f3 0f 1e fa 80 3d c5 3e 0e 00 00 74 13 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 54 c3 0f 1f 00 48 83 ec 28 48 89 54 24 18 48
     [41369.848117] RSP: 002b:00007fffee661738 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
     [41369.848119] RAX: ffffffffffffffda RBX: 0000570f66343bb0 RCX: 00007965df71a184
     [41369.848121] RDX: 0000000000040000 RSI: 0000570f66343bb0 RDI: 0000000000000003
     [41369.848122] RBP: 0000000000000003 R08: 0000000000000000 R09: 0000570f66343b20
     [41369.848122] R10: 0000000000000008 R11: 0000000000000202 R12: 0000000000000649
     [41369.848123] R13: 0000570f651f8b40 R14: 0000000000008000 R15: 0000570f6633bba0
     [41369.848125]  </TASK>
     [41369.848126] memory: usage 32768kB, limit 32768kB, failcnt 21239
     [41369.848126] swap: usage 2112kB, limit 9007199254740988kB, failcnt 0
     [41369.848127] Memory cgroup stats for /user.slice/user-1000.slice/user@1000.service/app.slice/run-u640.service:
     [41369.848174] anon 0
     [41369.848175] file 26927104
     [41369.848176] kernel 6615040
     [41369.848176] kernel_stack 32768
     [41369.848177] pagetables 122880
     [41369.848177] sec_pagetables 0
     [41369.848177] percpu 480
     [41369.848178] sock 0
     [41369.848178] vmalloc 0
     [41369.848178] shmem 0
     [41369.848179] zswap 312451
     [41369.848179] zswapped 1458176
     [41369.848179] file_mapped 0
     [41369.848180] file_dirty 26923008
     [41369.848180] file_writeback 0
     [41369.848180] swapcached 12288
     [41369.848181] anon_thp 0
     [41369.848181] file_thp 0
     [41369.848181] shmem_thp 0
     [41369.848182] inactive_anon 0
     [41369.848182] active_anon 12288
     [41369.848182] inactive_file 15908864
     [41369.848183] active_file 11014144
     [41369.848183] unevictable 0
     [41369.848183] slab_reclaimable 5963640
     [41369.848184] slab_unreclaimable 89048
     [41369.848184] slab 6052688
     [41369.848185] workingset_refault_anon 4031
     [41369.848185] workingset_refault_file 9236
     [41369.848185] workingset_activate_anon 691
     [41369.848186] workingset_activate_file 2553
     [41369.848186] workingset_restore_anon 691
     [41369.848186] workingset_restore_file 0
     [41369.848187] workingset_nodereclaim 0
     [41369.848187] pgscan 40473
     [41369.848187] pgsteal 20881
     [41369.848188] pgscan_kswapd 0
     [41369.848188] pgscan_direct 40473
     [41369.848188] pgscan_khugepaged 0
     [41369.848189] pgsteal_kswapd 0
     [41369.848189] pgsteal_direct 20881
     [41369.848190] pgsteal_khugepaged 0
     [41369.848190] pgfault 6019
     [41369.848190] pgmajfault 4033
     [41369.848191] pgrefill 30578988
     [41369.848191] pgactivate 2925
     [41369.848191] pgdeactivate 0
     [41369.848192] pglazyfree 0
     [41369.848192] pglazyfreed 0
     [41369.848192] zswpin 1520
     [41369.848193] zswpout 1141
     [41369.848193] thp_fault_alloc 0
     [41369.848193] thp_collapse_alloc 0
     [41369.848194] thp_swpout 0
     [41369.848194] thp_swpout_fallback 0
     [41369.848194] Tasks state (memory values in pages):
     [41369.848195] [  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name
     [41369.848195] [ 128825]  1000 128825     3449      864    65536      192           200 rsync
     [41369.848198] [ 128826]  1000 128826     3523      288    57344      288           200 rsync
     [41369.848199] oom-kill:constraint=CONSTRAINT_MEMCG,nodemask=(null),cpuset=/,mems_allowed=0,oom_memcg=/user.slice/user-1000.slice/user@1000.service/app.slice/run-u640.service,task_memcg=/user.slice/user-1000.slice/user@1000.service/app.slice/run-u640.service,task=rsync,pid=128825,uid=1000
     [41369.848207] Memory cgroup out of memory: Killed process 128825 (rsync) total-vm:13796kB, anon-rss:0kB, file-rss:3456kB, shmem-rss:0kB, UID:1000 pgtables:64kB oom_score_adj:200

---

Importantly, note that there appears to be no attempt to write back before 
declaring OOM -- file_writeback is 0 when file_dirty is 26923008. The issue is 
consistently reproducible (and thanks Johannes for looking at this with me).

On non-MGLRU, flushers are active and are making forward progress in preventing 
OOM.

This is writing to a slow disk with about ~10MiB/s available write speed, so 
the CPU and read speed is far faster than the write speed the disk 
can take.

Is this a known problem in MGLRU? If not, could you point me to where MGLRU 
tries to handle flusher wakeup on slow I/O? I didn't immediately find it.

Thanks,

Chris