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

[Marek Szyprowski]

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:

------------[ 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