Re: [PATCH v3] mm: kmem: add lockdep assertion to obj_cgroup_memcg

[Muchun Song]

> On Jul 31, 2024, at 02:52, Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> wrote:
> 
> On 25.07.2024 11:43, Muchun Song wrote:
>> The obj_cgroup_memcg() is supposed to safe to prevent the returned
>> memory cgroup from being freed only when the caller is holding the
>> rcu read lock or objcg_lock or cgroup_mutex. It is very easy to
>> ignore thoes conditions when users call some upper APIs which call
>> obj_cgroup_memcg() internally like mem_cgroup_from_slab_obj() (See
>> the link below). So it is better to add lockdep assertion to
>> obj_cgroup_memcg() to find those issues ASAP.
>> 
>> Because there is no user of obj_cgroup_memcg() holding objcg_lock
>> to make the returned memory cgroup safe, do not add objcg_lock
>> assertion (We should export objcg_lock if we really want to do).
>> Additionally, this is some internal implementation detail of memcg
>> and should not be accessible outside memcg code.
>> 
>> Some users like __mem_cgroup_uncharge() do not care the lifetime
>> of the returned memory cgroup, which just want to know if the
>> folio is charged to a memory cgroup, therefore, they do not need
>> to hold the needed locks. In which case, introduce a new helper
>> folio_memcg_charged() to do this. Compare it to folio_memcg(), it
>> could eliminate a memory access of objcg->memcg for kmem, actually,
>> a really small gain.
>> 
>> Link: https://lore.kernel.org/all/20240718083607.42068-1-songmuchun@xxxxxxxxxxxxx/
>> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> 
> This patch landed in today's linux-next as commit 230b2f1f31b9 ("mm: 
> kmem: add lockdep assertion to obj_cgroup_memcg"). I my tests I found 
> that it triggers the following warning on Debian bookworm/sid system 
> image running under QEMU RISCV64:

Thanks for your report.

I'd like to say excellent since it indeed indicates this patch works
well. Your report is actually a bug that I fixed it in [1] but not
related to this patch.

[1] https://lore.kernel.org/all/20240718083607.42068-1-songmuchun@xxxxxxxxxxxxx/

> 
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 1 at include/linux/memcontrol.h:373 
> mem_cgroup_from_slab_obj+0x13e/0x1ea
> Modules linked in:
> CPU: 0 UID: 0 PID: 1 Comm: systemd Not tainted 6.10.0+ #15154
> Hardware name: riscv-virtio,qemu (DT)
> epc : mem_cgroup_from_slab_obj+0x13e/0x1ea
>  ra : mem_cgroup_from_slab_obj+0x13c/0x1ea
> ...
> [<ffffffff80257256>] mem_cgroup_from_slab_obj+0x13e/0x1ea
> [<ffffffff801f0b3e>] list_lru_del_obj+0xa6/0xc2
> [<ffffffff8027c6c6>] d_lru_del+0x8c/0xa4
> [<ffffffff8027da60>] __dentry_kill+0x15e/0x17a
> [<ffffffff8027ec3c>] dput.part.0+0x242/0x3e6
> [<ffffffff8027edee>] dput+0xe/0x18
> [<ffffffff8027324c>] lookup_fast+0x80/0xce
> [<ffffffff80273e28>] walk_component+0x20/0x13c
> [<ffffffff802747e2>] path_lookupat+0x64/0x16c
> [<ffffffff80274bf4>] filename_lookup+0x76/0x122
> [<ffffffff80274d80>] user_path_at+0x30/0x4a
> [<ffffffff802d12bc>] __riscv_sys_name_to_handle_at+0x52/0x1d8
> [<ffffffff80b60324>] do_trap_ecall_u+0x14e/0x1da
> [<ffffffff80b6c546>] handle_exception+0xca/0xd6
> irq event stamp: 198187
> hardirqs last  enabled at (198187): [<ffffffff8028ca9e>] 
> lookup_mnt+0x186/0x308
> hardirqs last disabled at (198186): [<ffffffff8028ca74>] 
> lookup_mnt+0x15c/0x308
> softirqs last  enabled at (198172): [<ffffffff800e34f6>] 
> cgroup_apply_control_enable+0x1f6/0x2fc
> softirqs last disabled at (198170): [<ffffffff800e34d8>] 
> cgroup_apply_control_enable+0x1d8/0x2fc
> ---[ end trace 0000000000000000 ]---
> 
> Similar warning appears on ARM64 Debian bookworm system. Reverting it on 
> top of linux-next hides the issue, but I assume this is not the best way 
> to fix it.
> 
> I'm testing kernel built from riscv/defconfig with PROVE_LOCKING, 
> DEBUG_ATOMIC_SLEEP, DEBUG_DRIVER and DEBUG_DEVRES options enabled.
> 
>> ---
>> v3:
>>  - Use lockdep_assert_once(Vlastimil).
>> 
>> v2:
>>  - Remove mention of objcg_lock in obj_cgroup_memcg()(Shakeel Butt).
>> 
>>  include/linux/memcontrol.h | 20 +++++++++++++++++---
>>  mm/memcontrol.c            |  6 +++---
>>  2 files changed, 20 insertions(+), 6 deletions(-)
>> 
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index fc94879db4dff..95f823deafeca 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -360,11 +360,11 @@ static inline bool folio_memcg_kmem(struct folio *folio);
>>   * After the initialization objcg->memcg is always pointing at
>>   * a valid memcg, but can be atomically swapped to the parent memcg.
>>   *
>> - * The caller must ensure that the returned memcg won't be released:
>> - * e.g. acquire the rcu_read_lock or css_set_lock.
>> + * The caller must ensure that the returned memcg won't be released.
>>   */
>>  static inline struct mem_cgroup *obj_cgroup_memcg(struct obj_cgroup *objcg)
>>  {
>> + lockdep_assert_once(rcu_read_lock_held() || lockdep_is_held(&cgroup_mutex));
>>   return READ_ONCE(objcg->memcg);
>>  }
>> 
>> @@ -438,6 +438,19 @@ static inline struct mem_cgroup *folio_memcg(struct folio *folio)
>>   return __folio_memcg(folio);
>>  }
>> 
>> +/*
>> + * folio_memcg_charged - If a folio is charged to a memory cgroup.
>> + * @folio: Pointer to the folio.
>> + *
>> + * Returns true if folio is charged to a memory cgroup, otherwise returns false.
>> + */
>> +static inline bool folio_memcg_charged(struct folio *folio)
>> +{
>> + if (folio_memcg_kmem(folio))
>> + return __folio_objcg(folio) != NULL;
>> + return __folio_memcg(folio) != NULL;
>> +}
>> +
>>  /**
>>   * folio_memcg_rcu - Locklessly get the memory cgroup associated with a folio.
>>   * @folio: Pointer to the folio.
>> @@ -454,7 +467,6 @@ static inline struct mem_cgroup *folio_memcg_rcu(struct folio *folio)
>>   unsigned long memcg_data = READ_ONCE(folio->memcg_data);
>> 
>>   VM_BUG_ON_FOLIO(folio_test_slab(folio), folio);
>> - WARN_ON_ONCE(!rcu_read_lock_held());
>> 
>>   if (memcg_data & MEMCG_DATA_KMEM) {
>>   struct obj_cgroup *objcg;
>> @@ -463,6 +475,8 @@ static inline struct mem_cgroup *folio_memcg_rcu(struct folio *folio)
>>   return obj_cgroup_memcg(objcg);
>>   }
>> 
>> + WARN_ON_ONCE(!rcu_read_lock_held());
>> +
>>   return (struct mem_cgroup *)(memcg_data & ~OBJEXTS_FLAGS_MASK);
>>  }
>> 
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 622d4544edd24..3da0284573857 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -2366,7 +2366,7 @@ void mem_cgroup_cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
>> 
>>  static void commit_charge(struct folio *folio, struct mem_cgroup *memcg)
>>  {
>> - VM_BUG_ON_FOLIO(folio_memcg(folio), folio);
>> + VM_BUG_ON_FOLIO(folio_memcg_charged(folio), folio);
>>   /*
>>    * Any of the following ensures page's memcg stability:
>>    *
>> @@ -4617,7 +4617,7 @@ void __mem_cgroup_uncharge(struct folio *folio)
>>   struct uncharge_gather ug;
>> 
>>   /* Don't touch folio->lru of any random page, pre-check: */
>> - if (!folio_memcg(folio))
>> + if (!folio_memcg_charged(folio))
>>   return;
>> 
>>   uncharge_gather_clear(&ug);
>> @@ -4662,7 +4662,7 @@ void mem_cgroup_replace_folio(struct folio *old, struct folio *new)
>>   return;
>> 
>>   /* Page cache replacement: new folio already charged? */
>> - if (folio_memcg(new))
>> + if (folio_memcg_charged(new))
>>   return;
>> 
>>   memcg = folio_memcg(old);
> 
> Best regards
> -- 
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland