From 907aed48f65efeecf91575397e3d79335d93a466 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Tue, 31 Jul 2012 16:44:07 -0700 Subject: [PATCH] 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. This patch borrows the task flags from whatever process happens to be preempted by the softirq. It then modifies 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. This should be safe due to the following reasons Softirqs can run on multiple CPUs sure but the same task should not be executing the same softirq code. Neither should the softirq handler be preempted by any other softirq handler so the flags should not leak to an unrelated softirq. Softirqs re-enable hardware interrupts in __do_softirq() so can be preempted by hardware interrupts so PF_MEMALLOC is inherited by the hard IRQ. However, this is similar to a process in reclaim being preempted by a hardirq. While PF_MEMALLOC is set, gfp_to_alloc_flags() distinguishes between hard and soft irqs and avoids giving a hardirq the ALLOC_NO_WATERMARKS flag. If the softirq is deferred to ksoftirq then its flags may be used instead of a normal tasks but as the softirq cannot be preempted, the PF_MEMALLOC flag does not leak to other code by accident. [davem@davemloft.net: Document why PF_MEMALLOC is safe] Signed-off-by: Peter Zijlstra Signed-off-by: Mel Gorman Cc: David Miller Cc: Neil Brown Cc: Mike Christie Cc: Eric B Munson Cc: Eric Dumazet Cc: Sebastian Andrzej Siewior Cc: Mel Gorman Cc: Christoph Lameter Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- 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 865725adb9d3..c147e7024f11 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1894,6 +1894,13 @@ 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 671f9594e368..b73e681df09e 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 @@ asmlinkage void __do_softirq(void) 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 8f65abeb9ad6..cd5390f2f18d 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -2296,7 +2296,11 @@ 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; }