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Subject[PATCH v4] mm: introduce validity check on vm dirtiness settings
Date
we can find the logic in domain_dirty_limits() that
when dirty bg_thresh is bigger than dirty thresh,
bg_thresh will be set as thresh * 1 / 2.
if (bg_thresh >= thresh)
bg_thresh = thresh / 2;

But actually we can set vm background dirtiness bigger than
vm dirtiness successfully. This behavior may mislead us.
We'd better do this validity check at the beginning.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
Documentation/sysctl/vm.txt | 6 ++++
kernel/sysctl.c | 4 +--
mm/page-writeback.c | 80 ++++++++++++++++++++++++++++++++++++++++-----
3 files changed, 80 insertions(+), 10 deletions(-)

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 9baf66a..0bab85d 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -156,6 +156,9 @@ read.
Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
value lower than this limit will be ignored and the old configuration will be
retained.
+Note: the value of dirty_bytes also cannot be set lower than
+dirty_background_bytes or the amount of memory corresponding to
+dirty_background_ratio.

==============================================================

@@ -176,6 +179,9 @@ generating disk writes will itself start writing out dirty data.

The total available memory is not equal to total system memory.

+Note: dirty_ratio cannot be set lower than dirty_background_ratio or
+ratio corresponding to dirty_background_bytes.
+
==============================================================

dirty_writeback_centisecs
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 6648fbb..7b525cf 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1293,7 +1293,7 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write,
.maxlen = sizeof(dirty_background_ratio),
.mode = 0644,
.proc_handler = dirty_background_ratio_handler,
- .extra1 = &zero,
+ .extra1 = &one,
.extra2 = &one_hundred,
},
{
@@ -1310,7 +1310,7 @@ static int sysrq_sysctl_handler(struct ctl_table *table, int write,
.maxlen = sizeof(vm_dirty_ratio),
.mode = 0644,
.proc_handler = dirty_ratio_handler,
- .extra1 = &zero,
+ .extra1 = &one,
.extra2 = &one_hundred,
},
{
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 0b9c5cb..8dfd222 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -511,15 +511,61 @@ bool node_dirty_ok(struct pglist_data *pgdat)
return nr_pages <= limit;
}

+static bool vm_dirty_settings_valid(void)
+{
+ bool ret = true;
+ unsigned long bytes;
+
+ if (vm_dirty_ratio > 0) {
+ if (dirty_background_ratio >= vm_dirty_ratio) {
+ ret = false;
+ goto out;
+ }
+
+ bytes = global_dirtyable_memory() * PAGE_SIZE / 100 *
+ vm_dirty_ratio;
+ if (dirty_background_bytes >= bytes) {
+ ret = false;
+ goto out;
+ }
+ }
+
+ if (vm_dirty_bytes > 0) {
+ if (dirty_background_bytes >= vm_dirty_bytes) {
+ ret = false;
+ goto out;
+ }
+
+ bytes = global_dirtyable_memory() * PAGE_SIZE / 100 *
+ dirty_background_ratio;
+
+ if (bytes >= vm_dirty_bytes)
+ ret = false;
+ }
+
+out:
+ return ret;
+}
+
int dirty_background_ratio_handler(struct ctl_table *table, int write,
void __user *buffer, size_t *lenp,
loff_t *ppos)
{
int ret;
+ int old_ratio = dirty_background_ratio;

ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
- if (ret == 0 && write)
- dirty_background_bytes = 0;
+
+ /* needn't do validity check if the value is not different. */
+ if (ret == 0 && write && dirty_background_ratio != old_ratio) {
+ if (vm_dirty_settings_valid())
+ dirty_background_bytes = 0;
+ else {
+ dirty_background_ratio = old_ratio;
+ ret = -EINVAL;
+ }
+ }
+
return ret;
}

@@ -528,10 +574,18 @@ int dirty_background_bytes_handler(struct ctl_table *table, int write,
loff_t *ppos)
{
int ret;
+ unsigned long old_bytes = dirty_background_bytes;

ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
- if (ret == 0 && write)
- dirty_background_ratio = 0;
+ if (ret == 0 && write && dirty_background_bytes != old_bytes) {
+ if (vm_dirty_settings_valid())
+ dirty_background_ratio = 0;
+ else {
+ dirty_background_bytes = old_bytes;
+ ret = -EINVAL;
+ }
+ }
+
return ret;
}

@@ -544,8 +598,13 @@ int dirty_ratio_handler(struct ctl_table *table, int write,

ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
if (ret == 0 && write && vm_dirty_ratio != old_ratio) {
- writeback_set_ratelimit();
- vm_dirty_bytes = 0;
+ if (vm_dirty_settings_valid()) {
+ writeback_set_ratelimit();
+ vm_dirty_bytes = 0;
+ } else {
+ vm_dirty_ratio = old_ratio;
+ ret = -EINVAL;
+ }
}
return ret;
}
@@ -559,8 +618,13 @@ int dirty_bytes_handler(struct ctl_table *table, int write,

ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
- writeback_set_ratelimit();
- vm_dirty_ratio = 0;
+ if (vm_dirty_settings_valid()) {
+ writeback_set_ratelimit();
+ vm_dirty_ratio = 0;
+ } else {
+ vm_dirty_bytes = old_bytes;
+ ret = -EINVAL;
+ }
}
return ret;
}
--
1.8.3.1
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