Avoid to set less than two pages for vm_dirty_byte: this is necessary to avoid
potential division by 0 (like the following) in get_dirty_limits().
[ 49.951610] divide error: 0000 [#1] PREEMPT SMP
[ 49.952195] last sysfs file: /sys/devices/pci0000:00/0000:00:01.1/host0/target0:0:0/0:0:0:0/block/sda/uevent
[ 49.952195] CPU 1
[ 49.952195] Modules linked in: pcspkr
[ 49.952195] Pid: 3064, comm: dd Not tainted 2.6.30-rc3 #1
[ 49.952195] RIP: 0010:[<ffffffff802d39a9>] [<ffffffff802d39a9>] get_dirty_limits+0xe9/0x2c0
[ 49.952195] RSP: 0018:ffff88001de03a98 EFLAGS: 00010202
[ 49.952195] RAX: 00000000000000c0 RBX: ffff88001de03b80 RCX: 28f5c28f5c28f5c3
[ 49.952195] RDX: 0000000000000000 RSI: 00000000000000c0 RDI: 0000000000000000
[ 49.952195] RBP: ffff88001de03ae8 R08: 0000000000000000 R09: 0000000000000000
[ 49.952195] R10: ffff88001ddda9a0 R11: 0000000000000001 R12: 0000000000000001
[ 49.952195] R13: ffff88001fbc8218 R14: ffff88001de03b70 R15: ffff88001de03b78
[ 49.952195] FS: 00007fe9a435b6f0(0000) GS:ffff8800025d9000(0000) knlGS:0000000000000000
[ 49.952195] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 49.952195] CR2: 00007fe9a39ab000 CR3: 000000001de38000 CR4: 00000000000006e0
[ 49.952195] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 49.952195] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 49.952195] Process dd (pid: 3064, threadinfo ffff88001de02000, task ffff88001ddda250)
[ 49.952195] Stack:
[ 49.952195] ffff88001fa0de00 ffff88001f2dbd70 ffff88001f9fe800 000080b900000000
[ 49.952195] 00000000000000c0 ffff8800027a6100 0000000000000400 ffff88001fbc8218
[ 49.952195] 0000000000000000 0000000000000600 ffff88001de03bb8 ffffffff802d3ed7
[ 49.952195] Call Trace:
[ 49.952195] [<ffffffff802d3ed7>] balance_dirty_pages_ratelimited_nr+0x1d7/0x3f0
[ 49.952195] [<ffffffff80368f8e>] ? ext3_writeback_write_end+0x9e/0x120
[ 49.952195] [<ffffffff802cc7df>] generic_file_buffered_write+0x12f/0x330
[ 49.952195] [<ffffffff802cce8d>] __generic_file_aio_write_nolock+0x26d/0x460
[ 49.952195] [<ffffffff802cda32>] ? generic_file_aio_write+0x52/0xd0
[ 49.952195] [<ffffffff802cda49>] generic_file_aio_write+0x69/0xd0
[ 49.952195] [<ffffffff80365fa6>] ext3_file_write+0x26/0xc0
[ 49.952195] [<ffffffff803034d1>] do_sync_write+0xf1/0x140
[ 49.952195] [<ffffffff80290d1a>] ? get_lock_stats+0x2a/0x60
[ 49.952195] [<ffffffff80280730>] ? autoremove_wake_function+0x0/0x40
[ 49.952195] [<ffffffff8030411b>] vfs_write+0xcb/0x190
[ 49.952195] [<ffffffff803042d0>] sys_write+0x50/0x90
[ 49.952195] [<ffffffff8022ff6b>] system_call_fastpath+0x16/0x1b
[ 49.952195] Code: 00 00 00 2b 05 09 1c 17 01 48 89 c6 49 0f af f4 48 c1 ee 02 48 89 f0 48 f7 e1 48 89 d6 31 d2 48 c1 ee 02 48 0f af 75 d0 48 89 f0 <48> f7 f7 41 8b 95 ac 01 00 00 48 89 c7 49 0f af d4 48 c1 ea 02
[ 49.952195] RIP [<ffffffff802d39a9>] get_dirty_limits+0xe9/0x2c0
[ 49.952195] RSP <ffff88001de03a98>
[ 50.096523] ---[ end trace 008d7aa02f244d7b ]---
Signed-off-by: Andrea Righi <[email protected]>
---
mm/page-writeback.c | 6 ++++++
1 files changed, 6 insertions(+), 0 deletions(-)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 30351f0..9d6c427 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -213,6 +213,12 @@ int dirty_bytes_handler(struct ctl_table *table, int write,
int ret;
ret = proc_doulongvec_minmax(table, write, filp, buffer, lenp, ppos);
+ /*
+ * Do not allow to set less than two pages for vm_dirty_byte: this is
+ * necessary to avoid division by 0 in get_dirty_limits().
+ */
+ vm_dirty_bytes = max_t(typeof(vm_dirty_bytes),
+ vm_dirty_bytes, 2 * PAGE_SIZE);
if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
update_completion_period();
vm_dirty_ratio = 0;
--
1.6.0.4
On Wed, 2009-04-29 at 10:29 +0200, Andrea Righi wrote:
> Avoid to set less than two pages for vm_dirty_byte: this is necessary to avoid
> potential division by 0 (like the following) in get_dirty_limits().
isn't changing the .extra1 in the sysctl table a better fix?
> Signed-off-by: Andrea Righi <[email protected]>
> ---
> mm/page-writeback.c | 6 ++++++
> 1 files changed, 6 insertions(+), 0 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 30351f0..9d6c427 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -213,6 +213,12 @@ int dirty_bytes_handler(struct ctl_table *table, int write,
> int ret;
>
> ret = proc_doulongvec_minmax(table, write, filp, buffer, lenp, ppos);
> + /*
> + * Do not allow to set less than two pages for vm_dirty_byte: this is
> + * necessary to avoid division by 0 in get_dirty_limits().
> + */
> + vm_dirty_bytes = max_t(typeof(vm_dirty_bytes),
> + vm_dirty_bytes, 2 * PAGE_SIZE);
> if (ret == 0 && write && vm_dirty_bytes != old_bytes) {
> update_completion_period();
> vm_dirty_ratio = 0;
On Wed, 29 Apr 2009, Andrea Righi wrote:
> Avoid to set less than two pages for vm_dirty_byte: this is necessary to avoid
> potential division by 0 (like the following) in get_dirty_limits().
>
Where exactly is the divide by 0 in get_dirty_limits()?
Is this because we forced the background threshold to be half of the dirty
threshold when its setting is greater?
On Wed, Apr 29, 2009 at 10:44:36AM +0200, Peter Zijlstra wrote:
> On Wed, 2009-04-29 at 10:29 +0200, Andrea Righi wrote:
> > Avoid to set less than two pages for vm_dirty_byte: this is necessary to avoid
> > potential division by 0 (like the following) in get_dirty_limits().
>
> isn't changing the .extra1 in the sysctl table a better fix?
I thought about that and probably you're right. But, in the first case
if the user writes a value < 2*PAGE_SIZE vm_dirty_bytes is explicitly
set to 2*PAGE_SIZE. In the other case the value is just ignored.
Maybe the second case could generate some unexpected behaviours for
the user. For example, the user writes X < 2*PAGE_SIZE to
/proc/sys/vm/dirty_bytes, and could think that the system has started to
use this new configuration, but it isn't actually.
So, probably the best way is to change .extra1 and add a note in the
documentation (see below).
---
mm: prevent divide error for small values of vm_dirty_bytes
Avoid to set less than two pages for vm_dirty_bytes: this is necessary to
avoid potential division by 0 (like the following) in get_dirty_limits().
[ 49.951610] divide error: 0000 [#1] PREEMPT SMP
[ 49.952195] last sysfs file: /sys/devices/pci0000:00/0000:00:01.1/host0/target0:0:0/0:0:0:0/block/sda/uevent
[ 49.952195] CPU 1
[ 49.952195] Modules linked in: pcspkr
[ 49.952195] Pid: 3064, comm: dd Not tainted 2.6.30-rc3 #1
[ 49.952195] RIP: 0010:[<ffffffff802d39a9>] [<ffffffff802d39a9>] get_dirty_limits+0xe9/0x2c0
[ 49.952195] RSP: 0018:ffff88001de03a98 EFLAGS: 00010202
[ 49.952195] RAX: 00000000000000c0 RBX: ffff88001de03b80 RCX: 28f5c28f5c28f5c3
[ 49.952195] RDX: 0000000000000000 RSI: 00000000000000c0 RDI: 0000000000000000
[ 49.952195] RBP: ffff88001de03ae8 R08: 0000000000000000 R09: 0000000000000000
[ 49.952195] R10: ffff88001ddda9a0 R11: 0000000000000001 R12: 0000000000000001
[ 49.952195] R13: ffff88001fbc8218 R14: ffff88001de03b70 R15: ffff88001de03b78
[ 49.952195] FS: 00007fe9a435b6f0(0000) GS:ffff8800025d9000(0000) knlGS:0000000000000000
[ 49.952195] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 49.952195] CR2: 00007fe9a39ab000 CR3: 000000001de38000 CR4: 00000000000006e0
[ 49.952195] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 49.952195] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 49.952195] Process dd (pid: 3064, threadinfo ffff88001de02000, task ffff88001ddda250)
[ 49.952195] Stack:
[ 49.952195] ffff88001fa0de00 ffff88001f2dbd70 ffff88001f9fe800 000080b900000000
[ 49.952195] 00000000000000c0 ffff8800027a6100 0000000000000400 ffff88001fbc8218
[ 49.952195] 0000000000000000 0000000000000600 ffff88001de03bb8 ffffffff802d3ed7
[ 49.952195] Call Trace:
[ 49.952195] [<ffffffff802d3ed7>] balance_dirty_pages_ratelimited_nr+0x1d7/0x3f0
[ 49.952195] [<ffffffff80368f8e>] ? ext3_writeback_write_end+0x9e/0x120
[ 49.952195] [<ffffffff802cc7df>] generic_file_buffered_write+0x12f/0x330
[ 49.952195] [<ffffffff802cce8d>] __generic_file_aio_write_nolock+0x26d/0x460
[ 49.952195] [<ffffffff802cda32>] ? generic_file_aio_write+0x52/0xd0
[ 49.952195] [<ffffffff802cda49>] generic_file_aio_write+0x69/0xd0
[ 49.952195] [<ffffffff80365fa6>] ext3_file_write+0x26/0xc0
[ 49.952195] [<ffffffff803034d1>] do_sync_write+0xf1/0x140
[ 49.952195] [<ffffffff80290d1a>] ? get_lock_stats+0x2a/0x60
[ 49.952195] [<ffffffff80280730>] ? autoremove_wake_function+0x0/0x40
[ 49.952195] [<ffffffff8030411b>] vfs_write+0xcb/0x190
[ 49.952195] [<ffffffff803042d0>] sys_write+0x50/0x90
[ 49.952195] [<ffffffff8022ff6b>] system_call_fastpath+0x16/0x1b
[ 49.952195] Code: 00 00 00 2b 05 09 1c 17 01 48 89 c6 49 0f af f4 48 c1 ee 02 48 89 f0 48 f7 e1 48 89 d6 31 d2 48 c1 ee 02 48 0f af 75 d0 48 89 f0 <48> f7 f7 41 8b 95 ac 01 00 00 48 89 c7 49 0f af d4 48 c1 ea 02
[ 49.952195] RIP [<ffffffff802d39a9>] get_dirty_limits+0xe9/0x2c0
[ 49.952195] RSP <ffff88001de03a98>
[ 50.096523] ---[ end trace 008d7aa02f244d7b ]---
Signed-off-by: Andrea Righi <[email protected]>
---
Documentation/sysctl/vm.txt | 4 ++++
kernel/sysctl.c | 5 ++++-
2 files changed, 8 insertions(+), 1 deletions(-)
diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index 97c4b32..b716d33 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -90,6 +90,10 @@ will itself start writeback.
If dirty_bytes is written, dirty_ratio becomes a function of its value
(dirty_bytes / the amount of dirtyable system memory).
+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.
+
==============================================================
dirty_expire_centisecs
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index e3d2c7d..ea78fa1 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -103,6 +103,9 @@ static unsigned long one_ul = 1;
static int one_hundred = 100;
static int one_thousand = 1000;
+/* this is needed for the proc_doulongvec_minmax of vm_dirty_bytes */
+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;
@@ -1006,7 +1009,7 @@ static struct ctl_table vm_table[] = {
.mode = 0644,
.proc_handler = &dirty_bytes_handler,
.strategy = &sysctl_intvec,
- .extra1 = &one_ul,
+ .extra1 = &dirty_bytes_min,
},
{
.procname = "dirty_writeback_centisecs",
On Wed, Apr 29, 2009 at 02:26:11AM -0700, David Rientjes wrote:
> On Wed, 29 Apr 2009, Andrea Righi wrote:
>
> > Avoid to set less than two pages for vm_dirty_byte: this is necessary to avoid
> > potential division by 0 (like the following) in get_dirty_limits().
> >
>
> Where exactly is the divide by 0 in get_dirty_limits()?
in the evaluation of the BDI's share ratio:
...
if (bdi) {
u64 bdi_dirty;
long numerator, denominator;
/*
* Calculate this BDI's share of the dirty ratio.
*/
bdi_writeout_fraction(bdi, &numerator, &denominator);
bdi_dirty = (dirty * (100 - bdi_min_ratio)) / 100;
bdi_dirty *= numerator;
do_div(bdi_dirty, denominator);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
More exactly:
/home/righiandr/Software/linux/git/linux-2.6/mm/page-writeback.c:474
1186: 48 89 f0 mov %rsi,%rax
1189: 48 f7 f7 div %rdi
And looking at the trace:
[ 49.952195] RAX: 00000000000000c0 RBX: ffff88001de03b80 RCX: 28f5c28f5c28f5c3
[ 49.952195] RDX: 0000000000000000 RSI: 00000000000000c0 RDI: 0000000000000000
^^^^^^^^^^^^^^^^^^^^^
>
> Is this because we forced the background threshold to be half of the dirty
> threshold when its setting is greater?
No (see above).
-Andrea
On Wed, 29 Apr 2009, Andrea Righi wrote:
> On Wed, Apr 29, 2009 at 10:44:36AM +0200, Peter Zijlstra wrote:
> > On Wed, 2009-04-29 at 10:29 +0200, Andrea Righi wrote:
> > > Avoid to set less than two pages for vm_dirty_byte: this is necessary to avoid
> > > potential division by 0 (like the following) in get_dirty_limits().
> >
> > isn't changing the .extra1 in the sysctl table a better fix?
>
> I thought about that and probably you're right. But, in the first case
> if the user writes a value < 2*PAGE_SIZE vm_dirty_bytes is explicitly
> set to 2*PAGE_SIZE. In the other case the value is just ignored.
>
> Maybe the second case could generate some unexpected behaviours for
> the user. For example, the user writes X < 2*PAGE_SIZE to
> /proc/sys/vm/dirty_bytes, and could think that the system has started to
> use this new configuration, but it isn't actually.
>
> So, probably the best way is to change .extra1 and add a note in the
> documentation (see below).
>
This is only caused by bdi, not by the dirty ratio itself; shouldn't the
appropriate fix be in calc_period_shift()? In other words, allow
dirty thresholds to be less than 2 * PAGE_SIZE but prevent the bdi
fraction from having a zero denominator?
On Wed, 29 Apr 2009 11:34:51 +0200
Andrea Righi <[email protected]> wrote:
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -90,6 +90,10 @@ will itself start writeback.
> If dirty_bytes is written, dirty_ratio becomes a function of its value
> (dirty_bytes / the amount of dirtyable system memory).
>
> +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.
Well. This implies that the write to the procfs file would appear to
succeed. One hopes that the write would in fact return -EINVAL or
such?