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Commit 1babe183 authored by Wu Fengguang's avatar Wu Fengguang Committed by Linus Torvalds
Browse files

writeback: add comment to the dirty limit functions 



Document global_dirty_limits() and bdi_dirty_limit().

Signed-off-by: default avatarWu Fengguang <fengguang.wu@intel.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 16c4042f

mm/page-writeback.c

+28 −3
Original line number Diff line number Diff line
@@ -261,11 +261,18 @@ static inline void task_dirties_fraction(struct task_struct *tsk, 
}



/*

 * scale the dirty limit

 * task_dirty_limit - scale down dirty throttling threshold for one task

 *

 * task specific dirty limit:

 *

 *   dirty -= (dirty/8) * p_{t}

 *

 * To protect light/slow dirtying tasks from heavier/fast ones, we start

 * throttling individual tasks before reaching the bdi dirty limit.

 * Relatively low thresholds will be allocated to heavy dirtiers. So when

 * dirty pages grow large, heavy dirtiers will be throttled first, which will

 * effectively curb the growth of dirty pages. Light dirtiers with high enough

 * dirty threshold may never get throttled.

 */

static unsigned long task_dirty_limit(struct task_struct *tsk,

				       unsigned long bdi_dirty)
@@ -390,6 +397,15 @@ unsigned long determine_dirtyable_memory(void) 
	return x + 1;	/* Ensure that we never return 0 */

}



/**

 * global_dirty_limits - background-writeback and dirty-throttling thresholds

 *

 * Calculate the dirty thresholds based on sysctl parameters

 * - vm.dirty_background_ratio  or  vm.dirty_background_bytes

 * - vm.dirty_ratio             or  vm.dirty_bytes

 * The dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and

 * runtime tasks.

 */

void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)

{

	unsigned long background;
@@ -424,8 +440,17 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty) 
	*pdirty = dirty;

}



unsigned long bdi_dirty_limit(struct backing_dev_info *bdi,

			       unsigned long dirty)

/**

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

 *

 * Allocate high/low dirty limits to fast/slow devices, in order to prevent

 * - starving fast devices

 * - piling up dirty pages (that will take long time to sync) on slow devices

 *

 * The bdi's share of dirty limit will be adapting to its throughput and

 * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.

 */

unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)

{

	u64 bdi_dirty;

	long numerator, denominator;