Re: [PATCH] mm: prevent divide error for small values ofvm_dirty_bytes

From: Andrea Righi
Date: Wed Apr 29 2009 - 05:35:10 EST

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 <righi.andrea@xxxxxxxxx>
---
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",
--
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