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    SubjectRe: [Bug #10961] 2.6.26-rc: nfsd hangs for a few sec
    On (01/07/08 18:04), Alexander Beregalov didst pronounce:
    > 2008/7/1 Mel Gorman <mel@csn.ul.ie>:
    > > I still have no useful reaction to this. According to Christoph Hellwig,
    > > this lockup has been appearing since lockdep was introduced but for some
    > > reason is easier to trigger now. It bisected to the two-zonelist changes
    > > but it still looks like a red herring as I cannot see how reclaim has
    > > changed significantly as a result of that patch.
    >
    > Do you wait reaction from me? Can I help?
    > As I mentioned, the lockup does not happen when lockdep is disabled.
    >

    Sorry for the slow response Alexander.

    This bug is likely fixed by commit 494de90098784b8e2797598cefdd34188884ec2e
    which will be visible publicly later when maintenance on master.kernel.org
    finishes. I included it below for convenience.

    The lockdep warning still exists but it is a false positive and should be
    relatively hard to trigger again. It would be nice to have confirmation
    of this.

    commit 494de90098784b8e2797598cefdd34188884ec2e
    Author: Mel Gorman <mel@csn.ul.ie>
    Date: Thu Jul 3 05:27:51 2008 +0100

    Do not overwrite nr_zones on !NUMA when initialising zlcache_ptr

    The non-NUMA case of build_zonelist_cache() would initialize the
    zlcache_ptr for both node_zonelists[] to NULL.

    Which is problematic, since non-NUMA only has a single node_zonelists[]
    entry, and trying to zero the non-existent second one just overwrote the
    nr_zones field instead.

    As kswapd uses this value to determine what reclaim work is necessary,
    the result is that kswapd never reclaims. This causes processes to
    stall frequently in low-memory situations as they always direct reclaim.
    This patch initialises zlcache_ptr correctly.

    Signed-off-by: Mel Gorman <mel@csn.ul.ie>
    Tested-by: Dan Williams <dan.j.williams@intel.com>
    [ Simplified patch a bit ]
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    ---
    mm/page_alloc.c | 1 -
    1 files changed, 0 insertions(+), 1 deletions(-)

    diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    index 2f55295..f32fae3 100644
    --- a/mm/page_alloc.c
    +++ b/mm/page_alloc.c
    @@ -2328,7 +2328,6 @@ static void build_zonelists(pg_data_t *pgdat)
    static void build_zonelist_cache(pg_data_t *pgdat)
    {
    pgdat->node_zonelists[0].zlcache_ptr = NULL;
    - pgdat->node_zonelists[1].zlcache_ptr = NULL;
    }

    #endif /* CONFIG_NUMA */


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