[PATCH 01/31] mm: serialize access to min_free_kbytes

[PATCH 01/31] mm: serialize access to min_free_kbytes

[Suresh Jayaraman]

From: Peter Zijlstra <a.p.zijlstra@chello.nl> 

There is a small race between the procfs caller and the memory hotplug caller
of setup_per_zone_wmarks(). Not a big deal, but the next patch will add yet
another caller. Time to close the gap.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
---
 mm/page_alloc.c |   16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

Index: mmotm/mm/page_alloc.c
===================================================================
--- mmotm.orig/mm/page_alloc.c
+++ mmotm/mm/page_alloc.c
@@ -121,6 +121,7 @@ static char * const zone_names[MAX_NR_ZO
 	 "Movable",
 };
 
+static DEFINE_SPINLOCK(min_free_lock);
 int min_free_kbytes = 1024;
 
 unsigned long __meminitdata nr_kernel_pages;
@@ -4448,13 +4449,13 @@ static void setup_per_zone_lowmem_reserv
 }
 
 /**
- * setup_per_zone_wmarks - called when min_free_kbytes changes
+ * __setup_per_zone_wmarks - called when min_free_kbytes changes
  * or when memory is hot-{added|removed}
  *
  * Ensures that the watermark[min,low,high] values for each zone are set
  * correctly with respect to min_free_kbytes.
  */
-void setup_per_zone_wmarks(void)
+static void __setup_per_zone_wmarks(void)
 {
 	unsigned long pages_min = min_free_kbytes >> (PAGE_SHIFT - 10);
 	unsigned long lowmem_pages = 0;
@@ -4552,6 +4553,15 @@ static void __init setup_per_zone_inacti
 		calculate_zone_inactive_ratio(zone);
 }
 
+void setup_per_zone_wmarks(void)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&min_free_lock, flags);
+	__setup_per_zone_wmarks();
+	spin_unlock_irqrestore(&min_free_lock, flags);
+}
+
 /*
  * Initialise min_free_kbytes.
  *
@@ -4587,7 +4597,7 @@ static int __init init_per_zone_wmark_mi
 		min_free_kbytes = 128;
 	if (min_free_kbytes > 65536)
 		min_free_kbytes = 65536;
-	setup_per_zone_wmarks();
+	__setup_per_zone_wmarks();
 	setup_per_zone_lowmem_reserve();
 	setup_per_zone_inactive_ratio();
 	return 0;

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Re: [PATCH 01/31] mm: serialize access to min_free_kbytes

[David Rientjes]

On Thu, 1 Oct 2009, Suresh Jayaraman wrote:

> From: Peter Zijlstra <a.p.zijlstra@chello.nl> 
> 
> There is a small race between the procfs caller and the memory hotplug caller
> of setup_per_zone_wmarks(). Not a big deal, but the next patch will add yet
> another caller. Time to close the gap.
> 

By "next patch," you mean "mm: emegency pool" (patch 08/31)?

If so, can't you eliminate var_free_mutex entirely from that patch and 
take min_free_lock in adjust_memalloc_reserve() instead?

 [ __adjust_memalloc_reserve() would call __setup_per_zone_wmarks()
   under lock instead, now. ]

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Re: [PATCH 01/31] mm: serialize access to min_free_kbytes

[Neil Brown]

On Thursday October 1, rientjes@google.com wrote:
> On Thu, 1 Oct 2009, Suresh Jayaraman wrote:
> 
> > From: Peter Zijlstra <a.p.zijlstra@chello.nl> 
> > 
> > There is a small race between the procfs caller and the memory hotplug caller
> > of setup_per_zone_wmarks(). Not a big deal, but the next patch will add yet
> > another caller. Time to close the gap.
> > 
> 
> By "next patch," you mean "mm: emegency pool" (patch 08/31)?

:-)  It is always safer to say "a subsequent patch", isn't it....

> 
> If so, can't you eliminate var_free_mutex entirely from that patch and 
> take min_free_lock in adjust_memalloc_reserve() instead?

adjust_memalloc_reserve does a test alloc/free cycle under a lock.
That cannot be done under a spin-lock, it must be a mutex.
So I don't think you can eliminate var_free_mutex.

Thanks,
NeilBrown

> 
>  [ __adjust_memalloc_reserve() would call __setup_per_zone_wmarks()
>    under lock instead, now. ]

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