Subject: [PATCH -mmotm 07/30] mm: allow PF_MEMALLOC from softirq context

From: Xiaotian Feng <[email-masked]>
Date: Tue, 13 Jul 2010 06:18:10 -0400

>>From 32f474cffc76551ddb792454845bd473634219b5 Mon Sep 17 00:00:00 2001
From: Xiaotian Feng <[email-masked]>
Date: Tue, 13 Jul 2010 10:41:53 +0800
Subject: [PATCH 07/30] mm: allow PF_MEMALLOC from softirq context

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 <[email-masked]>
Signed-off-by: Suresh Jayaraman <[email-masked]>
Signed-off-by: Xiaotian Feng <[email-masked]>
---
 include/linux/sched.h |    7 +++++++
 kernel/softirq.c      |    3 +++
 mm/page_alloc.c       |    7 ++++---
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1f25798..85b74b0 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1765,6 +1765,13 @@ 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 int set_cpus_allowed_ptr(struct task_struct *p,
 				const struct cpumask *new_mask);
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 07b4f1b..0770e78 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -194,6 +194,8 @@ asmlinkage void __do_softirq(void)
 	__u32 pending;
 	int max_restart = MAX_SOFTIRQ_RESTART;
 	int cpu;
+	unsigned long pflags = current->flags;
+	current->flags &= ~PF_MEMALLOC;
 
 	pending = local_softirq_pending();
 	account_system_vtime(current);
@@ -246,6 +248,7 @@ restart:
 
 	account_system_vtime(current);
 	_local_bh_enable();
+	tsk_restore_flags(current, pflags, PF_MEMALLOC);
 }
 
 #ifndef __ARCH_HAS_DO_SOFTIRQ
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b9989c5..4b1fa10 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1928,9 +1928,10 @@ int gfp_to_alloc_flags(gfp_t gfp_mask)
 		alloc_flags |= ALLOC_HARDER;
 
 	if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
-		if (!in_interrupt() &&
-		    ((p->flags & PF_MEMALLOC) ||
-		     unlikely(test_thread_flag(TIF_MEMDIE))))
+		if (!in_irq() && (p->flags & PF_MEMALLOC))
+			alloc_flags |= ALLOC_NO_WATERMARKS;
+		else if (!in_interrupt() &&
+				unlikely(test_thread_flag(TIF_MEMDIE)))
 			alloc_flags |= ALLOC_NO_WATERMARKS;
 	}
 
-- 
1.7.1.1

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