Re: meminfo Committed_AS underflows

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

fix stupid mistake.

Changelog:
	since v1
		- change the type of commited value from ulong to long


=========================
Subject: [PATCH v2] fix Commited_AS underflow

Dave Hansen reported committed_AS field can underfolow.

>         # while true; do cat /proc/meminfo  | grep _AS; sleep 1; done | uniq -c
>               1 Committed_AS: 18446744073709323392 kB
>              11 Committed_AS: 18446744073709455488 kB
>               6 Committed_AS:    35136 kB
>               5 Committed_AS: 18446744073709454400 kB
>               7 Committed_AS:    35904 kB
>               3 Committed_AS: 18446744073709453248 kB
>               2 Committed_AS:    34752 kB
>               9 Committed_AS: 18446744073709453248 kB
>               8 Committed_AS:    34752 kB
>               3 Committed_AS: 18446744073709320960 kB
>               7 Committed_AS: 18446744073709454080 kB
>               3 Committed_AS: 18446744073709320960 kB
>               5 Committed_AS: 18446744073709454080 kB
>               6 Committed_AS: 18446744073709320960 kB

Because NR_CPU can be greater than 1000. and meminfo_proc_show()
doesn't have underflow check.

this patch have two change.

1. Change NR_CPU to nr_online_cpus()
   vm_acct_memory() isn't fast-path. then cpumask_weight() calculation
   isn't so expensive and the parameter for scalability issue should
   consider number of _physical_ cpus. not theoretical maximum number.
2. Add under-flow check to meminfo_proc_show().
   Almost field in /proc/meminfo have underflow check. but Committed_AS
   is significant exeption.
   it should do.

Reported-by: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
---
 fs/proc/meminfo.c |    4 +++-
 mm/swap.c         |    2 +-
 2 files changed, 4 insertions(+), 2 deletions(-)

Index: b/fs/proc/meminfo.c
===================================================================
--- a/fs/proc/meminfo.c
+++ b/fs/proc/meminfo.c
@@ -22,7 +22,7 @@ void __attribute__((weak)) arch_report_m
 static int meminfo_proc_show(struct seq_file *m, void *v)
 {
 	struct sysinfo i;
-	unsigned long committed;
+	long committed;
 	unsigned long allowed;
 	struct vmalloc_info vmi;
 	long cached;
@@ -36,6 +36,8 @@ static int meminfo_proc_show(struct seq_
 	si_meminfo(&i);
 	si_swapinfo(&i);
 	committed = atomic_long_read(&vm_committed_space);
+	if (committed < 0)
+		committed = 0;
 	allowed = ((totalram_pages - hugetlb_total_pages())
 		* sysctl_overcommit_ratio / 100) + total_swap_pages;
 
Index: b/mm/swap.c
===================================================================
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -519,7 +519,7 @@ EXPORT_SYMBOL(pagevec_lookup_tag);
  * We tolerate a little inaccuracy to avoid ping-ponging the counter between
  * CPUs
  */
-#define ACCT_THRESHOLD	max(16, NR_CPUS * 2)
+#define ACCT_THRESHOLD	max_t(long, 16, num_online_cpus() * 2)
 
 static DEFINE_PER_CPU(long, committed_space);
 


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