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

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)

+{
+	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;

 	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);
 }
 

...