linux-kernel - Re: [PATCH 2/2] mm, oom_reaper: do not mmput synchronously from the oom reaper context

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Date:	Fri, 20 May 2016 08:16:59 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm, oom_reaper: do not mmput synchronously from the
 oom reaper context

On Fri 20-05-16 10:30:53, Minchan Kim wrote:
> Forking new thread because my comment is not related to this patch's
> purpose but found a thing during reading this patch.
> 
> On Tue, Apr 26, 2016 at 04:04:30PM +0200, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > Tetsuo has properly noted that mmput slow path might get blocked waiting
> > for another party (e.g. exit_aio waits for an IO). If that happens the
> > oom_reaper would be put out of the way and will not be able to process
> > next oom victim. We should strive for making this context as reliable
> > and independent on other subsystems as much as possible.
> > 
> > Introduce mmput_async which will perform the slow path from an async
> > (WQ) context. This will delay the operation but that shouldn't be a
> > problem because the oom_reaper has reclaimed the victim's address space
> > for most cases as much as possible and the remaining context shouldn't
> > bind too much memory anymore. The only exception is when mmap_sem
> > trylock has failed which shouldn't happen too often.
> > 
> > The issue is only theoretical but not impossible.
> 
> The mmput_async is used for only OOM reaper which is enabled on CONFIG_MMU.
> So until someone who want to use mmput_async in !CONFIG_MMU come out,
> we could save sizeof(struct work_struct) per mm in !CONFIG_MMU.

You are right. What about the following?
---
>From 8f8a34bf00882bfc0b557ed79e0e9e956ac9d217 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.com>
Date: Fri, 20 May 2016 08:14:39 +0200
Subject: [PATCH] mmotm:
 mm-oom_reaper-do-not-mmput-synchronously-from-the-oom-reaper-context-fix

mmput_async is currently used only from the oom_reaper which is defined
only for CONFIG_MMU. We can save work_struct in mm_struct for
!CONFIG_MMU.

Reported-by: Minchan Kim <minchan@...nel.org>
Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 include/linux/mm_types.h | 2 ++
 include/linux/sched.h    | 2 ++
 kernel/fork.c            | 2 ++
 3 files changed, 6 insertions(+)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index ab142ace96f3..a16dcb2efca4 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -514,7 +514,9 @@ struct mm_struct {
 #ifdef CONFIG_HUGETLB_PAGE
 	atomic_long_t hugetlb_usage;
 #endif
+#ifdef CONFIG_MMU
 	struct work_struct async_put_work;
+#endif
 };
 
 static inline void mm_init_cpumask(struct mm_struct *mm)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index df8778e72211..11b31ded65cf 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2604,10 +2604,12 @@ static inline void mmdrop(struct mm_struct * mm)
 
 /* mmput gets rid of the mappings and all user-space */
 extern void mmput(struct mm_struct *);
+#ifdef CONFIG_MMU
 /* same as above but performs the slow path from the async kontext. Can
  * be called from the atomic context as well
  */
 extern void mmput_async(struct mm_struct *);
+#endif
 
 /* Grab a reference to a task's mm, if it is not already going away */
 extern struct mm_struct *get_task_mm(struct task_struct *task);
diff --git a/kernel/fork.c b/kernel/fork.c
index e1dc6b02ac8b..1e3dc3af6845 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -732,6 +732,7 @@ void mmput(struct mm_struct *mm)
 }
 EXPORT_SYMBOL_GPL(mmput);
 
+#ifdef CONFIG_MMU
 static void mmput_async_fn(struct work_struct *work)
 {
 	struct mm_struct *mm = container_of(work, struct mm_struct, async_put_work);
@@ -745,6 +746,7 @@ void mmput_async(struct mm_struct *mm)
 		schedule_work(&mm->async_put_work);
 	}
 }
+#endif
 
 /**
  * set_mm_exe_file - change a reference to the mm's executable file
-- 
2.8.1


-- 
Michal Hocko
SUSE Labs