Re: [PATCH v5 5/6] mm/memcg: Protect memcg_stock with a local_lock_t

Michal Hocko
2022-02-28T08:06:53Z

On Sat 26-02-22 21:41:43, Sebastian Andrzej Siewior wrote:
> The members of the per-CPU structure memcg_stock_pcp are protected by
> disabling interrupts. This is not working on PREEMPT_RT because it
> creates atomic context in which actions are performed which require
> preemptible context. One example is obj_cgroup_release().
>
> The IRQ-disable sections can be replaced with local_lock_t which
> preserves the explicit disabling of interrupts while keeps the code
> preemptible on PREEMPT_RT.
>
> drain_obj_stock() drops a reference on obj_cgroup which leads to an invocation
> of obj_cgroup_release() if it is the last object. This in turn leads to
> recursive locking of the local_lock_t. To avoid this, obj_cgroup_release() is
> invoked outside of the locked section.
>
> obj_cgroup_uncharge_pages() can be invoked with the local_lock_t acquired and
> without it. This will lead later to a recursion in refill_stock(). To
> avoid the locking recursion provide obj_cgroup_uncharge_pages_locked()
> which uses the locked version of refill_stock().
>
> - Replace disabling interrupts for memcg_stock with a local_lock_t.
>
> - Let drain_obj_stock() return the old struct obj_cgroup which is passed
> to obj_cgroup_put() outside of the locked section.
>
> - Provide obj_cgroup_uncharge_pages_locked() which uses the locked
> version of refill_stock() to avoid recursive locking in
> drain_obj_stock().
>
> Link: https://lkml.kernel.org/r/20220209014709.GA26885@xsang-OptiPlex-9020
> Reported-by: kernel test robot <oliver.sang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>

I thought I have already acked this one. Anyway
Acked-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>

> ---
> mm/memcontrol.c | 59 +++++++++++++++++++++++++++++++------------------
> 1 file changed, 38 insertions(+), 21 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 4d049b4691afd..6439b0089d392 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2135,6 +2135,7 @@ void unlock_page_memcg(struct page *page)
> }
>
> struct memcg_stock_pcp {
> + local_lock_t stock_lock;
> struct mem_cgroup *cached; /* this never be root cgroup */
> unsigned int nr_pages;
>
> @@ -2150,18 +2151,21 @@ struct memcg_stock_pcp {
> unsigned long flags;
> #define FLUSHING_CACHED_CHARGE 0
> };
> -static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
> +static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock) = {
> + .stock_lock = INIT_LOCAL_LOCK(stock_lock),
> +};
> static DEFINE_MUTEX(percpu_charge_mutex);
>
> #ifdef CONFIG_MEMCG_KMEM
> -static void drain_obj_stock(struct memcg_stock_pcp *stock);
> +static struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock);
> static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
> struct mem_cgroup *root_memcg);
> static void memcg_account_kmem(struct mem_cgroup *memcg, int nr_pages);
>
> #else
> -static inline void drain_obj_stock(struct memcg_stock_pcp *stock)
> +static inline struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock)
> {
> + return NULL;
> }
> static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
> struct mem_cgroup *root_memcg)
> @@ -2193,7 +2197,7 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> if (nr_pages > MEMCG_CHARGE_BATCH)
> return ret;
>
> - local_irq_save(flags);
> + local_lock_irqsave(&memcg_stock.stock_lock, flags);
>
> stock = this_cpu_ptr(&memcg_stock);
> if (memcg == stock->cached && stock->nr_pages >= nr_pages) {
> @@ -2201,7 +2205,7 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> ret = true;
> }
>
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
>
> return ret;
> }
> @@ -2230,6 +2234,7 @@ static void drain_stock(struct memcg_stock_pcp *stock)
> static void drain_local_stock(struct work_struct *dummy)
> {
> struct memcg_stock_pcp *stock;
> + struct obj_cgroup *old = NULL;
> unsigned long flags;
>
> /*
> @@ -2237,14 +2242,16 @@ static void drain_local_stock(struct work_struct *dummy)
> * drain_stock races is that we always operate on local CPU stock
> * here with IRQ disabled
> */
> - local_irq_save(flags);
> + local_lock_irqsave(&memcg_stock.stock_lock, flags);
>
> stock = this_cpu_ptr(&memcg_stock);
> - drain_obj_stock(stock);
> + old = drain_obj_stock(stock);
> drain_stock(stock);
> clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
>
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
> + if (old)
> + obj_cgroup_put(old);
> }
>
> /*
> @@ -2271,9 +2278,9 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
> {
> unsigned long flags;
>
> - local_irq_save(flags);
> + local_lock_irqsave(&memcg_stock.stock_lock, flags);
> __refill_stock(memcg, nr_pages);
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
> }
>
> /*
> @@ -3100,10 +3107,11 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
> enum node_stat_item idx, int nr)
> {
> struct memcg_stock_pcp *stock;
> + struct obj_cgroup *old = NULL;
> unsigned long flags;
> int *bytes;
>
> - local_irq_save(flags);
> + local_lock_irqsave(&memcg_stock.stock_lock, flags);
> stock = this_cpu_ptr(&memcg_stock);
>
> /*
> @@ -3112,7 +3120,7 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
> * changes.
> */
> if (stock->cached_objcg != objcg) {
> - drain_obj_stock(stock);
> + old = drain_obj_stock(stock);
> obj_cgroup_get(objcg);
> stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes)
> ? atomic_xchg(&objcg->nr_charged_bytes, 0) : 0;
> @@ -3156,7 +3164,9 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
> if (nr)
> mod_objcg_mlstate(objcg, pgdat, idx, nr);
>
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
> + if (old)
> + obj_cgroup_put(old);
> }
>
> static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
> @@ -3165,7 +3175,7 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
> unsigned long flags;
> bool ret = false;
>
> - local_irq_save(flags);
> + local_lock_irqsave(&memcg_stock.stock_lock, flags);
>
> stock = this_cpu_ptr(&memcg_stock);
> if (objcg == stock->cached_objcg && stock->nr_bytes >= nr_bytes) {
> @@ -3173,17 +3183,17 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
> ret = true;
> }
>
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
>
> return ret;
> }
>
> -static void drain_obj_stock(struct memcg_stock_pcp *stock)
> +static struct obj_cgroup *drain_obj_stock(struct memcg_stock_pcp *stock)
> {
> struct obj_cgroup *old = stock->cached_objcg;
>
> if (!old)
> - return;
> + return NULL;
>
> if (stock->nr_bytes) {
> unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
> @@ -3233,8 +3243,12 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock)
> stock->cached_pgdat = NULL;
> }
>
> - obj_cgroup_put(old);
> stock->cached_objcg = NULL;
> + /*
> + * The `old' objects needs to be released by the caller via
> + * obj_cgroup_put() outside of memcg_stock_pcp::stock_lock.
> + */
> + return old;
> }
>
> static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
> @@ -3255,14 +3269,15 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
> bool allow_uncharge)
> {
> struct memcg_stock_pcp *stock;
> + struct obj_cgroup *old = NULL;
> unsigned long flags;
> unsigned int nr_pages = 0;
>
> - local_irq_save(flags);
> + local_lock_irqsave(&memcg_stock.stock_lock, flags);
>
> stock = this_cpu_ptr(&memcg_stock);
> if (stock->cached_objcg != objcg) { /* reset if necessary */
> - drain_obj_stock(stock);
> + old = drain_obj_stock(stock);
> obj_cgroup_get(objcg);
> stock->cached_objcg = objcg;
> stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes)
> @@ -3276,7 +3291,9 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
> stock->nr_bytes &= (PAGE_SIZE - 1);
> }
>
> - local_irq_restore(flags);
> + local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
> + if (old)
> + obj_cgroup_put(old);
>
> if (nr_pages)
> obj_cgroup_uncharge_pages(objcg, nr_pages);
> --
> 2.35.1

--
Michal Hocko
SUSE Labs