[PATCH 05/17] mm: allow PF_MEMALLOC from softirq context

[Mel Gorman <mgorman@suse.de>]

This is needed to allow network softirq packet processing to make
use of 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.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 include/linux/sched.h |    7 +++++++
 kernel/softirq.c      |    9 +++++++++
 mm/page_alloc.c       |    6 +++++-
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 81a173c..b5efaf4 100644
--- 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 *task,
+				unsigned long orig_flags, unsigned long flags)
+{
+	task->flags &= ~flags;
+	task->flags |= orig_flags & flags;
+}
+
 #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..b73e681 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -210,6 +210,14 @@ asmlinkage void __do_softirq(void)
 	__u32 pending;
 	int max_restart = MAX_SOFTIRQ_RESTART;
 	int cpu;
+	unsigned long old_flags = current->flags;
+
+	/*
+	 * Mask out PF_MEMALLOC s current task context is borrowed for the
+	 * softirq. A softirq handled such as network RX might set PF_MEMALLOC
+	 * again if the socket is related to swap
+	 */
+	current->flags &= ~PF_MEMALLOC;
 
 	pending = local_softirq_pending();
 	account_system_vtime(current);
@@ -265,6 +273,7 @@ restart:
 
 	account_system_vtime(current);
 	__local_bh_enable(SOFTIRQ_OFFSET);
+	tsk_restore_flags(current, old_flags, PF_MEMALLOC);
 }
 
 #ifndef __ARCH_HAS_DO_SOFTIRQ
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6040520..3738596 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2211,7 +2211,11 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
 	if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
 		if (gfp_mask & __GFP_MEMALLOC)
 			alloc_flags |= ALLOC_NO_WATERMARKS;
-		else if (likely(!(gfp_mask & __GFP_NOMEMALLOC)) && !in_interrupt())
+		else if (in_serving_softirq() && (current->flags & PF_MEMALLOC))
+			alloc_flags |= ALLOC_NO_WATERMARKS;
+		else if (!in_interrupt() &&
+				((current->flags & PF_MEMALLOC) ||
+				 unlikely(test_thread_flag(TIF_MEMDIE))))
 			alloc_flags |= ALLOC_NO_WATERMARKS;
 	}
 
-- 
1.7.9.2

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