The change is virtually a no-op for the majority users that use the default 10/20 background/dirty ratios. For others don't know why they are setting background ratio close enough to dirty ratio. Someone must set background ratio equal to dirty ratio, but no one seems to notice or complain that it's then silently halved under the hood.. The other solution is to return -EIO when setting a too large background threshold or a too small dirty threshold. However that could possibly break some disordered usage scenario, eg. echo 10 > /proc/sys/vm/dirty_ratio echo 5 > /proc/sys/vm/dirty_background_ratio The first echo will fail because the background ratio is still 10. Such order dependent behavior seems disgusting for end users. CC: Peter Zijlstra Signed-off-by: Wu Fengguang --- mm/page-writeback.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) --- linux-next.orig/mm/page-writeback.c 2011-03-02 17:04:16.000000000 +0800 +++ linux-next/mm/page-writeback.c 2011-03-02 17:06:17.000000000 +0800 @@ -422,8 +422,14 @@ void global_dirty_limits(unsigned long * else background = (dirty_background_ratio * available_memory) / 100; - if (background >= dirty) - background = dirty / 2; + /* + * Ensure at least 1/4 gap between background and dirty thresholds, so + * that when dirty throttling starts at (background + dirty)/2, it's + * below or at the entrance of the soft dirty throttle scope. + */ + if (background > dirty - dirty / (DIRTY_SCOPE / 2)) + background = dirty - dirty / (DIRTY_SCOPE / 2); + tsk = current; if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) { background += background / 4; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/