Messages in this thread | | | Date | Tue, 1 May 2012 15:08:13 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 05/16] mm: allow PF_MEMALLOC from softirq context |
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On Mon, 16 Apr 2012 13:16:52 +0100 Mel Gorman <mgorman@suse.de> wrote:
> This is needed to allow network softirq packet processing to make > use of PF_MEMALLOC.
hm, why? You just added __GFP_MEMALLOC so we don't need to futz with PF_MEMALLOC?
> Currently softirq context cannot use PF_MEMALLOC due to it not being > associated with a task, and therefore not having task flags to fiddle > with - thus the gfp to alloc flag mapping ignores the task flags when > in interrupts (hard or soft) context. > > Allowing softirqs to make use of PF_MEMALLOC therefore requires some > trickery. We basically borrow the task flags from whatever process > happens to be preempted by the softirq. > > So we modify the gfp to alloc flags mapping to not exclude task flags > in softirq context, and modify the softirq code to save, clear and > restore the PF_MEMALLOC flag. > > The save and clear, ensures the preempted task's PF_MEMALLOC flag > doesn't leak into the softirq. The restore ensures a softirq's > PF_MEMALLOC flag cannot leak back into the preempted process. > > ... > > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1913,6 +1913,13 @@ static inline void rcu_copy_process(struct task_struct *p) > > #endif > > +static inline void tsk_restore_flags(struct task_struct *p, > + unsigned long pflags, unsigned long mask)
The naming is poor.
p -> "tsk" or "task" pflags -> "old_flags" mask -> "flags"
> +{ > + p->flags &= ~mask; > + p->flags |= pflags & mask; > +} > + > #ifdef CONFIG_SMP > extern void do_set_cpus_allowed(struct task_struct *p, > const struct cpumask *new_mask); > diff --git a/kernel/softirq.c b/kernel/softirq.c > index 671f959..d349caa 100644 > --- a/kernel/softirq.c > +++ b/kernel/softirq.c > @@ -210,6 +210,8 @@ asmlinkage void __do_softirq(void) > __u32 pending; > int max_restart = MAX_SOFTIRQ_RESTART; > int cpu; > + unsigned long pflags = current->flags;
"old_flags"
> + current->flags &= ~PF_MEMALLOC;
The line before this one would be a suitable place for a comment!
> pending = local_softirq_pending(); > account_system_vtime(current); > @@ -265,6 +267,7 @@ restart: > > account_system_vtime(current); > __local_bh_enable(SOFTIRQ_OFFSET); > + tsk_restore_flags(current, pflags, PF_MEMALLOC); > } > > ... >
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