If the percpu_pagelist_fraction sysctl is set by the user, it is
impossible to restore it to the kernel default since the user cannot
write 0 to the sysctl.
This patch allows the user to write 0 to restore the default behavior.
It still requires a fraction equal to or larger than 8, however, as stated
by the documentation for sanity. If a value in the range [1, 7] is
written, the sysctl will return EINVAL.
This also fixes a division by zero identified by Oleg that occurs if a
write() occurs with zero length and the value hasn't been changed before
(so that percpu_pagelist_fraction is still 0).
Reported-by: Oleg Drokin <gr...@linuxhacker.ru>
Cc: sta...@vger.kernel.org
Signed-off-by: David Rientjes <rient...@google.com>
---
Documentation/sysctl/vm.txt | 3 ++-
kernel/sysctl.c | 3 +--
mm/page_alloc.c | 30 +++++++++++++++++++++---------
3 files changed, 24 insertions(+), 12 deletions(-)
diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -702,7 +702,8 @@ The batch value of each per cpu pagelist is also updated as
a result. It is
set to pcp->high/4. The upper limit of batch is (PAGE_SHIFT * 8)
The initial value is zero. Kernel does not use this value at boot time to set
-the high water marks for each per cpu page list.
+the high water marks for each per cpu page list. If the user writes '0' to
this
+sysctl, it will revert to this default behavior.
==============================================================
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -136,7 +136,6 @@ static unsigned long dirty_bytes_min = 2 * PAGE_SIZE;
/* this is needed for the proc_dointvec_minmax for [fs_]overflow UID and GID */
static int maxolduid = 65535;
static int minolduid;
-static int min_percpu_pagelist_fract = 8;
static int ngroups_max = NGROUPS_MAX;
static const int cap_last_cap = CAP_LAST_CAP;
@@ -1305,7 +1304,7 @@ static struct ctl_table vm_table[] = {
.maxlen = sizeof(percpu_pagelist_fraction),
.mode = 0644,
.proc_handler = percpu_pagelist_fraction_sysctl_handler,
- .extra1 = &min_percpu_pagelist_fract,
+ .extra1 = &zero,
},
#ifdef CONFIG_MMU
{
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -69,6 +69,7 @@
/* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
static DEFINE_MUTEX(pcp_batch_high_lock);
+#define MIN_PERCPU_PAGELIST_FRACTION (8)
#ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
DEFINE_PER_CPU(int, numa_node);
@@ -4107,7 +4108,7 @@ static void __meminit zone_init_free_lists(struct zone
*zone)
memmap_init_zone((size), (nid), (zone), (start_pfn), MEMMAP_EARLY)
#endif
-static int __meminit zone_batchsize(struct zone *zone)
+static int zone_batchsize(struct zone *zone)
{
#ifdef CONFIG_MMU
int batch;
@@ -4223,8 +4224,8 @@ static void pageset_set_high(struct per_cpu_pageset *p,
pageset_update(&p->pcp, high, batch);
}
-static void __meminit pageset_set_high_and_batch(struct zone *zone,
- struct per_cpu_pageset *pcp)
+static void pageset_set_high_and_batch(struct zone *zone,
+ struct per_cpu_pageset *pcp)
{
if (percpu_pagelist_fraction)
pageset_set_high(pcp,
@@ -5849,21 +5850,32 @@ int lowmem_reserve_ratio_sysctl_handler(ctl_table
*table, int write,
int percpu_pagelist_fraction_sysctl_handler(ctl_table *table, int write,
void __user *buffer, size_t *length, loff_t *ppos)
{
+ const int old_percpu_pagelist_fraction = percpu_pagelist_fraction;
struct zone *zone;
- unsigned int cpu;
int ret;
ret = proc_dointvec_minmax(table, write, buffer, length, ppos);
- if (!write || (ret < 0))
+ if (!write || ret < 0)
return ret;
+ /* Sanity checking to avoid pcp imbalance */
+ if (percpu_pagelist_fraction &&
+ percpu_pagelist_fraction < MIN_PERCPU_PAGELIST_FRACTION) {
+ percpu_pagelist_fraction = old_percpu_pagelist_fraction;
+ return -EINVAL;
+ }
+
+ /* No change? */
+ if (percpu_pagelist_fraction == old_percpu_pagelist_fraction)
+ return 0;
+
mutex_lock(&pcp_batch_high_lock);
for_each_populated_zone(zone) {
- unsigned long high;
- high = zone->managed_pages / percpu_pagelist_fraction;
+ unsigned int cpu;
+
for_each_possible_cpu(cpu)
- pageset_set_high(per_cpu_ptr(zone->pageset, cpu),
- high);
+ pageset_set_high_and_batch(zone,
+ per_cpu_ptr(zone->pageset, cpu));
}
mutex_unlock(&pcp_batch_high_lock);
return 0;
--
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