md: improve the is_mddev_idle test fix

Don't use 'unsigned' variable to track sync vs non-sync IO, as the only thing
we want to do with them is a signed comparison, and fix up the comment which
had become quite wrong.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 22 insertions, 13 deletions

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 3d5a6beb333..bae42331182 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -5091,7 +5091,7 @@ static int is_mddev_idle(mddev_t *mddev)
         mdk_rdev_t * rdev;
         struct list_head *tmp;
         int idle;
-        unsigned long curr_events;
+        long curr_events;
 
         idle = 1;
         ITERATE_RDEV(mddev,rdev,tmp) {
@@ -5099,20 +5099,29 @@ static int is_mddev_idle(mddev_t *mddev)
                 curr_events = disk_stat_read(disk, sectors[0]) + 
                                 disk_stat_read(disk, sectors[1]) - 
                                 atomic_read(&disk->sync_io);
-                /* The difference between curr_events and last_events
-                 * will be affected by any new non-sync IO (making
-                 * curr_events bigger) and any difference in the amount of
-                 * in-flight syncio (making current_events bigger or smaller)
-                 * The amount in-flight is currently limited to
-                 * 32*64K in raid1/10 and 256*PAGE_SIZE in raid5/6
-                 * which is at most 4096 sectors.
-                 * These numbers are fairly fragile and should be made
-                 * more robust, probably by enforcing the
-                 * 'window size' that md_do_sync sort-of uses.
+                /* sync IO will cause sync_io to increase before the disk_stats
+                 * as sync_io is counted when a request starts, and
+                 * disk_stats is counted when it completes.
+                 * So resync activity will cause curr_events to be smaller than
+                 * when there was no such activity.
+                 * non-sync IO will cause disk_stat to increase without
+                 * increasing sync_io so curr_events will (eventually)
+                 * be larger than it was before.  Once it becomes
+                 * substantially larger, the test below will cause
+                 * the array to appear non-idle, and resync will slow
+                 * down.
+                 * If there is a lot of outstanding resync activity when
+                 * we set last_event to curr_events, then all that activity
+                 * completing might cause the array to appear non-idle
+                 * and resync will be slowed down even though there might
+                 * not have been non-resync activity.  This will only
+                 * happen once though.  'last_events' will soon reflect
+                 * the state where there is little or no outstanding
+                 * resync requests, and further resync activity will
+                 * always make curr_events less than last_events.
                  *
-                 * Note: the following is an unsigned comparison.
                  */
-                if ((long)curr_events - (long)rdev->last_events > 4096) {
+                if (curr_events - rdev->last_events > 4096) {
                         rdev->last_events = curr_events;
                         idle = 0;
                 }