> 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
