Subject: Re: [PATCH 05/16] mm: allow PF_MEMALLOC from softirq context

From: Andrew Morton <[email-masked]>
Date: Tue, 1 May 2012 15:08:13 -0700

On Mon, 16 Apr 2012 13:16:52 +0100
Mel Gorman <[email-masked]> 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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