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

Peter Zijlstra
2007-09-29T11:48:01Z

On Sat, 2007-09-29 at 19:04 +0800, 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?

In another sub-thread:

> It's works on .23-rc8-mm2 with out any problems.
>
> "dd" process does not hang any more.
>
> Thanks for all the help.
>
> Cheers
> --Chakri

So the per-bdi dirty patches that are in -mm already fix the problem.

> ===
> 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;
>

On the patch itself, not sure if it would have been enough. As soon as
there is a single dirty inode on the list one would get caught in the
same problem as before.

That is, if NFS_dirty+NFS_unstable+NFS_writeback > dirty_limit this
break won't fix it.