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SubjectRe: meminfo Committed_AS underflows
Date

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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