On Mon 21-02-22 19:25:39, 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@xxxxxxxxx> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Thank you, this looks much more straightforward. I am not super fan of the requirement to call obj_cgroup_put oudside of drain_obj_stock but I can live with that. A comment in drain_obj_stock to call that out explicitly would be nice (can be done as a follow up and have Andrew fold it in if there is no new resubmission). Acked-by: Michal Hocko <mhocko@xxxxxxxx> > --- > mm/memcontrol.c | 55 ++++++++++++++++++++++++++++++------------------- > 1 file changed, 34 insertions(+), 21 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index cd3950256519d..3d7ccb104374c 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2128,6 +2128,7 @@ 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; > > @@ -2143,18 +2144,21 @@ 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) > @@ -2186,7 +2190,7 @@ 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) { > @@ -2194,7 +2198,7 @@ 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; > } > @@ -2223,6 +2227,7 @@ 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; > > /* > @@ -2230,14 +2235,16 @@ 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); > } > > /* > @@ -2264,9 +2271,9 @@ 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); > } > > /* > @@ -3093,10 +3100,11 @@ 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); > > /* > @@ -3105,7 +3113,7 @@ 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; > @@ -3149,7 +3157,9 @@ 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) > @@ -3158,7 +3168,7 @@ 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) { > @@ -3166,17 +3176,17 @@ 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; > @@ -3226,8 +3236,8 @@ static void drain_obj_stock(struct memcg_stock_pcp *stock) > stock->cached_pgdat = NULL; > } > > - obj_cgroup_put(old); > stock->cached_objcg = NULL; > + return old; > } > > static bool obj_stock_flush_required(struct memcg_stock_pcp *stock, > @@ -3248,14 +3258,15 @@ 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) > @@ -3269,7 +3280,9 @@ 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
