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    Subject[PATCH 12/27] nfs: lower writeback threshold proportionally to dirty threshold
    nfs_congestion_kb is to control the max allowed writeback and in-commit
    pages. It's not reasonable for them to outnumber dirty and to-commit
    pages. So each of them should not take more than 1/4 dirty threshold.

    Considering that nfs_init_writepagecache() is called on fresh boot,
    at the time dirty_thresh is much higher than the real dirty limit after
    lots of user space memory consumptions, use 1/8 instead.

    We might update nfs_congestion_kb when global dirty limit is changed
    at runtime, but whatever, do it simple first.

    CC: Trond Myklebust <Trond.Myklebust@netapp.com>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    fs/nfs/write.c | 13 +++++++++++++
    mm/page-writeback.c | 1 +
    2 files changed, 14 insertions(+)

    --- linux-next.orig/fs/nfs/write.c 2011-03-03 14:04:01.000000000 +0800
    +++ linux-next/fs/nfs/write.c 2011-03-03 14:04:01.000000000 +0800
    @@ -1651,6 +1651,9 @@ out:

    int __init nfs_init_writepagecache(void)
    {
    + unsigned long background_thresh;
    + unsigned long dirty_thresh;
    +
    nfs_wdata_cachep = kmem_cache_create("nfs_write_data",
    sizeof(struct nfs_write_data),
    0, SLAB_HWCACHE_ALIGN,
    @@ -1688,6 +1691,16 @@ int __init nfs_init_writepagecache(void)
    if (nfs_congestion_kb > 256*1024)
    nfs_congestion_kb = 256*1024;

    + /*
    + * Limit to 1/8 dirty threshold, so that writeback+in_commit pages
    + * won't overnumber dirty+to_commit pages.
    + */
    + global_dirty_limits(&background_thresh, &dirty_thresh);
    + dirty_thresh <<= PAGE_SHIFT - 10;
    +
    + if (nfs_congestion_kb > dirty_thresh / 8)
    + nfs_congestion_kb = dirty_thresh / 8;
    +
    return 0;
    }

    --- linux-next.orig/mm/page-writeback.c 2011-03-03 14:04:01.000000000 +0800
    +++ linux-next/mm/page-writeback.c 2011-03-03 14:04:01.000000000 +0800
    @@ -431,6 +431,7 @@ void global_dirty_limits(unsigned long *
    *pbackground = background;
    *pdirty = dirty;
    }
    +EXPORT_SYMBOL_GPL(global_dirty_limits);

    /**
    * bdi_dirty_limit - @bdi's share of dirty throttling threshold



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