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Message-ID: <3c4b978b-b1fe-42d2-b1a7-a58609433f3c@samsung.com>
Date: Tue, 30 Jul 2024 20:52:04 +0200
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: Muchun Song <songmuchun@...edance.com>, hannes@...xchg.org,
mhocko@...nel.org, roman.gushchin@...ux.dev, shakeel.butt@...ux.dev,
muchun.song@...ux.dev, akpm@...ux-foundation.org, vbabka@...nel.org
Cc: cgroups@...r.kernel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm: kmem: add lockdep assertion to obj_cgroup_memcg
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@bytedance.com/
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
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
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