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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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