mm-vmscan-throttle-reclaim-if-encountering-too-many-dirty-pages-under-writeback-update

This patch expands on a comment on how we throttle from reclaim context.
It should be merged with
mm-vmscan-throttle-reclaim-if-encountering-too-many-dirty-pages-under-writeback.patch

Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Andrew Morton <>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c3aefa7fb600469c4c4693404517e00e836c3ab0..7b0573f33a27a0b47c9e11277394a5b8284c275b 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1538,11 +1538,27 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
        putback_lru_pages(zone, sc, nr_anon, nr_file, &page_list);
 
        /*
-        * If we have encountered a high number of dirty pages under writeback
-        * then we are reaching the end of the LRU too quickly and global
-        * limits are not enough to throttle processes due to the page
-        * distribution throughout zones. Scale the number of dirty pages that
-        * must be under writeback before being throttled to priority.
+        * If reclaim is isolating dirty pages under writeback, it implies
+        * that the long-lived page allocation rate is exceeding the page
+        * laundering rate. Either the global limits are not being effective
+        * at throttling processes due to the page distribution throughout
+        * zones or there is heavy usage of a slow backing device. The
+        * only option is to throttle from reclaim context which is not ideal
+        * as there is no guarantee the dirtying process is throttled in the
+        * same way balance_dirty_pages() manages.
+        *
+        * This scales the number of dirty pages that must be under writeback
+        * before throttling depending on priority. It is a simple backoff
+        * function that has the most effect in the range DEF_PRIORITY to
+        * DEF_PRIORITY-2 which is the priority reclaim is considered to be
+        * in trouble and reclaim is considered to be in trouble.
+        *
+        * DEF_PRIORITY   100% isolated pages must be PageWriteback to throttle
+        * DEF_PRIORITY-1  50% must be PageWriteback
+        * DEF_PRIORITY-2  25% must be PageWriteback, kswapd in trouble
+        * ...
+        * DEF_PRIORITY-6 For SWAP_CLUSTER_MAX isolated pages, throttle if any
+        *                     isolated page is PageWriteback
         */
        if (nr_writeback && nr_writeback >= (nr_taken >> (DEF_PRIORITY-priority)))
                wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);