md/raid5: separate out the easy conditions in need_this_block.

Some of the conditions in need_this_block have very straight
forward motivation.  Separate those out and document them.

Signed-off-by: NeilBrown <neilb@suse.de>

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 5a980fe..21c1e79 100644 (file)
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -2903,14 +2903,34 @@ static int need_this_block(struct stripe_head *sh, struct stripe_head_state *s,
        struct r5dev *fdev[2] = { &sh->dev[s->failed_num[0]],
                                  &sh->dev[s->failed_num[1]] };
 
-       if (!test_bit(R5_LOCKED, &dev->flags) &&
-           !test_bit(R5_UPTODATE, &dev->flags) &&
-           (dev->toread ||
-            (dev->towrite && !test_bit(R5_OVERWRITE, &dev->flags)) ||
-            s->syncing || s->expanding ||
-            (s->replacing && want_replace(sh, disk_idx)) ||
-            (s->failed >= 1 && fdev[0]->toread) ||
-            (s->failed >= 2 && fdev[1]->toread) ||
+
+       if (test_bit(R5_LOCKED, &dev->flags) ||
+           test_bit(R5_UPTODATE, &dev->flags))
+               /* No point reading this as we already have it or have
+                * decided to get it.
+                */
+               return 0;
+
+       if (dev->toread ||
+           (dev->towrite && !test_bit(R5_OVERWRITE, &dev->flags)))
+               /* We need this block to directly satisfy a request */
+               return 1;
+
+       if (s->syncing || s->expanding ||
+           (s->replacing && want_replace(sh, disk_idx)))
+               /* When syncing, or expanding we read everything.
+                * When replacing, we need the replaced block.
+                */
+               return 1;
+
+       if ((s->failed >= 1 && fdev[0]->toread) ||
+           (s->failed >= 2 && fdev[1]->toread))
+               /* If we want to read from a failed device, then
+                * we need to actually read every other device.
+                */
+               return 1;
+
+       if (
             (sh->raid_conf->level <= 5 && s->failed && fdev[0]->towrite &&
              (!test_bit(R5_Insync, &dev->flags) || test_bit(STRIPE_PREREAD_ACTIVE, &sh->state)) &&
              !test_bit(R5_OVERWRITE, &fdev[0]->flags)) ||
@@ -2919,7 +2939,7 @@ static int need_this_block(struct stripe_head *sh, struct stripe_head_state *s,
              && s->failed && s->to_write &&
              (s->to_write - s->non_overwrite <
               sh->raid_conf->raid_disks - sh->raid_conf->max_degraded) &&
-             (!test_bit(R5_Insync, &dev->flags) || test_bit(STRIPE_PREREAD_ACTIVE, &sh->state)))))
+             (!test_bit(R5_Insync, &dev->flags) || test_bit(STRIPE_PREREAD_ACTIVE, &sh->state))))
                return 1;
        return 0;
 }