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SubjectRe: [PATCH] writeback: avoid possible balance_dirty_pages() lockup on a light-load bdi
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On Tue, 2007-10-02 at 10:00 +0800, Fengguang Wu wrote:
> On Mon, Oct 01, 2007 at 11:57:34AM -0400, Chuck Ebbert wrote:
> > On 09/29/2007 07:04 AM, Fengguang Wu wrote:
...
>
> (expecting real world confirmations...)
>
> Here is a new safer version. It's more ugly though.
>
> ---
> writeback: avoid possible balance_dirty_pages() lockup on a 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 available 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
>
> 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'.)
>
> But the newly introduced 'break' could make the nr_writeback drift away
> above the dirty limit. The workaround is to limit the error under 1MB.
>
> Cc: Chuck Ebbert <cebbert@redhat.com>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
> ---
> mm/page-writeback.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> --- linux-2.6.22.orig/mm/page-writeback.c
> +++ linux-2.6.22/mm/page-writeback.c
> @@ -250,6 +250,11 @@ static void balance_dirty_pages(struct a
> pages_written += write_chunk - wbc.nr_to_write;
> if (pages_written >= write_chunk)
> break; /* We've done our duty */
> + if (list_empty(&mapping->host->i_sb->s_dirty) &&
> + list_empty(&mapping->host->i_sb->s_io) &&
> + nr_reclaimable + global_page_state(NR_WRITEBACK) <=
> + dirty_thresh + (1 << (20-PAGE_CACHE_SHIFT)))
> + break;
> }
> congestion_wait(WRITE, HZ/10);
> }

I've been testing 2.6.23-rc9 + this patch all morning but have just seen
a lockup. As usual it happened just after a large file copy finished
and while nr_dirty is still large. I'm sorry to say I didn't have a
serial console running so I don't have an other info. I will try again
and see if I can capture some more data.

I did notice that at the beginning of my tests the dirty blocks are
written back more quickly than usual

nr_dirty count after the copy finished and then 60 seconds later :-
after copy +60 seconds
73520 0
73533 0
68554 1

but after several iterations of my testcase & just before the lockup
68560 57165
71974 62896

which is about the same as a unpatched kernel.

Richard


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