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>
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 3d5a6be..bae4233 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -5091,7 +5091,7 @@
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 @@
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;
}