Re: [PATCH -next 01/13] md: remove rdev flag 'RemoveSynchronized'

Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> · Tue, 15 Aug 2023 09:04:54 +0800

Hi,

在 2023/08/15 5:36, NeilBrown 写道:
On Mon, 14 Aug 2023, Song Liu wrote:
+ Neil

RemoveSynchronized was added by Neil not too long ago. I wonder whether
we still need it in some cases (large raid10 losing a large set of disks).

We have customers with thousands of members in a raid10.  They do
sometimes need to remove half of them.  Dropping RemoveSynchronized
would significantly increase the time it takes to do that.

Removing an rdev is not often a hot path.  But sometimes it can be.


Thanks for the explanation, and soory that I didn't mention that in this
commit message, the way that rdev is protected through rcu is not safe,
and refer to the implementation of 'q_usage_counter' in block layer, I'm
planing to refactor the way that rdev is protected and rcu is not needed
anymore:

1. refactor mddev_suspend(), and make sure it's called by all the caller
of remove_and_add_spares().
2. factor out a helper md_array_enter() from md_handle_request, this
helper will grab 'active_io' and wait for mddev_suspend() to be done.
3. in the fastpath, before accessing rdev, make sure md_array_enter() is
called.

Thanks,
Kuai

NeilBrown



Thanks,
Song


On Thu, Aug 3, 2023 at 9:27 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:

From: Yu Kuai <yukuai3@xxxxxxxxxx>

When multiple rdevs are removed from array in remove_and_add_spares(),
which is only possible from daemon thread(user can only remove one rdev
through ioctl/sysfs at one time), the flag is used to avoid calling
synchronize_rcu() multiple times for each rdev.

However, remove rdev from daemon thread really is super cold path, while
removing n rdevs, it doesn't matter sync rcu n times or just one time.
Hence remove this flag and make code simpler.

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
  drivers/md/md-multipath.c | 15 +++++++--------
  drivers/md/md.c           | 26 ++------------------------
  drivers/md/md.h           |  5 -----
  drivers/md/raid1.c        | 15 +++++++--------
  drivers/md/raid10.c       | 15 +++++++--------
  drivers/md/raid5.c        | 15 +++++++--------
  6 files changed, 30 insertions(+), 61 deletions(-)

diff --git a/drivers/md/md-multipath.c b/drivers/md/md-multipath.c
index d22276870283..9e4ddd5240cd 100644
--- a/drivers/md/md-multipath.c
+++ b/drivers/md/md-multipath.c
@@ -258,14 +258,13 @@ static int multipath_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
                         goto abort;
                 }
                 p->rdev = NULL;
-               if (!test_bit(RemoveSynchronized, &rdev->flags)) {
-                       synchronize_rcu();
-                       if (atomic_read(&rdev->nr_pending)) {
-                               /* lost the race, try later */
-                               err = -EBUSY;
-                               p->rdev = rdev;
-                               goto abort;
-                       }
+
+               synchronize_rcu();
+               if (atomic_read(&rdev->nr_pending)) {
+                       /* lost the race, try later */
+                       err = -EBUSY;
+                       p->rdev = rdev;
+                       goto abort;
                 }
                 err = md_integrity_register(mddev);
         }
diff --git a/drivers/md/md.c b/drivers/md/md.c
index a3d98273b295..cd7ac1dee3b8 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -9155,7 +9155,6 @@ static int remove_and_add_spares(struct mddev *mddev,
         struct md_rdev *rdev;
         int spares = 0;
         int removed = 0;
-       bool remove_some = false;

         if (this && test_bit(MD_RECOVERY_RUNNING, &mddev->recovery))
                 /* Mustn't remove devices when resync thread is running */
@@ -9165,28 +9164,9 @@ static int remove_and_add_spares(struct mddev *mddev,
                 if ((this == NULL || rdev == this) &&
                     rdev->raid_disk >= 0 &&
                     !test_bit(Blocked, &rdev->flags) &&
-                   test_bit(Faulty, &rdev->flags) &&
+                   !test_bit(In_sync, &rdev->flags) &&
+                   !test_bit(Journal, &rdev->flags) &&
                     atomic_read(&rdev->nr_pending)==0) {
-                       /* Faulty non-Blocked devices with nr_pending == 0
-                        * never get nr_pending incremented,
-                        * never get Faulty cleared, and never get Blocked set.
-                        * So we can synchronize_rcu now rather than once per device
-                        */
-                       remove_some = true;
-                       set_bit(RemoveSynchronized, &rdev->flags);
-               }
-       }
-
-       if (remove_some)
-               synchronize_rcu();
-       rdev_for_each(rdev, mddev) {
-               if ((this == NULL || rdev == this) &&
-                   rdev->raid_disk >= 0 &&
-                   !test_bit(Blocked, &rdev->flags) &&
-                   ((test_bit(RemoveSynchronized, &rdev->flags) ||
-                    (!test_bit(In_sync, &rdev->flags) &&
-                     !test_bit(Journal, &rdev->flags))) &&
-                   atomic_read(&rdev->nr_pending)==0)) {
                         if (mddev->pers->hot_remove_disk(
                                     mddev, rdev) == 0) {
                                 sysfs_unlink_rdev(mddev, rdev);
@@ -9195,8 +9175,6 @@ static int remove_and_add_spares(struct mddev *mddev,
                                 removed++;
                         }
                 }
-               if (remove_some && test_bit(RemoveSynchronized, &rdev->flags))
-                       clear_bit(RemoveSynchronized, &rdev->flags);
         }

         if (removed && mddev->kobj.sd)
diff --git a/drivers/md/md.h b/drivers/md/md.h
index 8ae957480976..b25b6d061372 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -189,11 +189,6 @@ enum flag_bits {
                                  * than other devices in the array
                                  */
         ClusterRemove,
-       RemoveSynchronized,     /* synchronize_rcu() was called after
-                                * this device was known to be faulty,
-                                * so it is safe to remove without
-                                * another synchronize_rcu() call.
-                                */
         ExternalBbl,            /* External metadata provides bad
                                  * block management for a disk
                                  */
diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 23d211969565..acb6d6542619 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -1859,15 +1859,14 @@ static int raid1_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
                         goto abort;
                 }
                 p->rdev = NULL;
-               if (!test_bit(RemoveSynchronized, &rdev->flags)) {
-                       synchronize_rcu();
-                       if (atomic_read(&rdev->nr_pending)) {
-                               /* lost the race, try later */
-                               err = -EBUSY;
-                               p->rdev = rdev;
-                               goto abort;
-                       }
+               synchronize_rcu();
+               if (atomic_read(&rdev->nr_pending)) {
+                       /* lost the race, try later */
+                       err = -EBUSY;
+                       p->rdev = rdev;
+                       goto abort;
                 }
+
                 if (conf->mirrors[conf->raid_disks + number].rdev) {
                         /* We just removed a device that is being replaced.
                          * Move down the replacement.  We drain all IO before
diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 7704a4c7f469..64dd5cb6133e 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -2247,15 +2247,14 @@ static int raid10_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
                 goto abort;
         }
         *rdevp = NULL;
-       if (!test_bit(RemoveSynchronized, &rdev->flags)) {
-               synchronize_rcu();
-               if (atomic_read(&rdev->nr_pending)) {
-                       /* lost the race, try later */
-                       err = -EBUSY;
-                       *rdevp = rdev;
-                       goto abort;
-               }
+       synchronize_rcu();
+       if (atomic_read(&rdev->nr_pending)) {
+               /* lost the race, try later */
+               err = -EBUSY;
+               *rdevp = rdev;
+               goto abort;
         }
+
         if (p->replacement) {
                 /* We must have just cleared 'rdev' */
                 p->rdev = p->replacement;
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 4cdb35e54251..37d9865b180a 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -8267,15 +8267,14 @@ static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
                 goto abort;
         }
         *rdevp = NULL;
-       if (!test_bit(RemoveSynchronized, &rdev->flags)) {
-               lockdep_assert_held(&mddev->reconfig_mutex);
-               synchronize_rcu();
-               if (atomic_read(&rdev->nr_pending)) {
-                       /* lost the race, try later */
-                       err = -EBUSY;
-                       rcu_assign_pointer(*rdevp, rdev);
-               }
+       lockdep_assert_held(&mddev->reconfig_mutex);
+       synchronize_rcu();
+       if (atomic_read(&rdev->nr_pending)) {
+               /* lost the race, try later */
+               err = -EBUSY;
+               rcu_assign_pointer(*rdevp, rdev);
         }
+
         if (!err) {
                 err = log_modify(conf, rdev, false);
                 if (err)
--
2.39.2




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