Re: A unresponsive file system can hang all I/O in the system on linux-2.6.23-rc6 (dirty_thresh problem?)

Chuck Ebbert
2007-10-01T15:57:34Z

On 09/29/2007 07:04 AM, Fengguang Wu wrote:
> On Thu, Sep 27, 2007 at 11:32:36PM -0700, Chakri n wrote:
>> Hi,
>>
>> In my testing, a unresponsive file system can hang all I/O in the system.
>> This is not seen in 2.4.
>>
>> I started 20 threads doing I/O on a NFS share. They are just doing 4K
>> writes in a loop.
>>
>> Now I stop NFS server hosting the NFS share and start a
>> "dd" process to write a file on local EXT3 file system.
>>
>> # dd if=/dev/zero of=/tmp/x count=1000
>>
>> This process never progresses.
>
> Peter, do you think this patch will help?
>
> ===
> writeback: avoid possible balance_dirty_pages() lockup on light-load bdi
>
> On a busy-writing system, a writer could be hold up infinitely on a
> light-load device. It will be trying to sync more than enough dirty data.
>
> The problem case:
>
> 0. sda/nr_dirty >= dirty_limit;
> sdb/nr_dirty == 0
> 1. dd writes 32 pages on sdb
> 2. balance_dirty_pages() blocks dd, and tries to write 6MB.
> 3. it never gets there: there's only 128KB dirty data.
> 4. dd may be blocked for a loooong time as long as sda is overloaded
>
> Fix it by returning on 'zero dirty inodes' in the current bdi.
> (In fact there are slight differences between 'dirty inodes' and 'dirty pages'.
> But there is no available counters for 'dirty pages'.)
>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
> ---
> mm/page-writeback.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> --- linux-2.6.22.orig/mm/page-writeback.c
> +++ linux-2.6.22/mm/page-writeback.c
> @@ -227,6 +227,9 @@ static void balance_dirty_pages(struct a
> if (nr_reclaimable + global_page_state(NR_WRITEBACK) <=
> dirty_thresh)
> break;
> + if (list_empty(&mapping->host->i_sb->s_dirty) &&
> + list_empty(&mapping->host->i_sb->s_io))
> + break;
>
> if (!dirty_exceeded)
> dirty_exceeded = 1;
>

This looks better than the other candidate to fix the problem. Are we going
to fix 2.6.23 before release? Multiple people have reported this problem now...