Re: [PATCH v2 2/2] btrfs: zoned: automatically reclaim zones

[Anand Jain]

On 19/03/2021 18:48, Johannes Thumshirn wrote:

When a file gets deleted on a zoned file system, the space freed is not
returned back into the block group's free space, but is migrated to
zone_unusable.

As this zone_unusable space is behind the current write pointer it is not
possible to use it for new allocations. In the current implementation a
zone is reset once all of the block group's space is accounted as zone
unusable.

This behaviour can lead to premature ENOSPC errors on a busy file system.

Instead of only reclaiming the zone once it is completely unusable,
kick off a reclaim job once the amount of unusable bytes exceeds a user
configurable threshold between 51% and 100%. It can be set per mounted
filesystem via the sysfs tunable bg_reclaim_threshold which is set to 75%
per default.

Similar to reclaiming unused block groups, these dirty block groups are
added to a to_reclaim list and then on a transaction commit, the reclaim
process is triggered but after we deleted unused block groups, which will
free space for the relocation process.

Still, in the code below, I don't see the zone write pointer reset of
the zone_unusable done here. Where does that happen? Or what did I miss?

Signed-off-by: Johannes Thumshirn <johannes.thumsh...@wdc.com>
---

AFAICT sysfs_create_files() does not have the ability to provide a is_visible
callback, so the bg_reclaim_threshold sysfs file is visible for non zoned
filesystems as well, even though only for zoned filesystems we're adding block
groups to the reclaim list. I'm all ears for a approach that is sensible in
this regard.

All per mounted filesystem kobjects remain visible, even if it doesn't
make sense in some configs. So it is consistent with the rest.

Thanks, Anand

  fs/btrfs/block-group.c       | 84 ++++++++++++++++++++++++++++++++++++
  fs/btrfs/block-group.h       |  2 +
  fs/btrfs/ctree.h             |  3 ++
  fs/btrfs/disk-io.c           | 11 +++++
  fs/btrfs/free-space-cache.c  |  9 +++-
  fs/btrfs/sysfs.c             | 35 +++++++++++++++
  fs/btrfs/volumes.c           |  2 +-
  fs/btrfs/volumes.h           |  1 +
  include/trace/events/btrfs.h | 12 ++++++
  9 files changed, 157 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index 9ae3ac96a521..af9026795ddd 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -1485,6 +1485,80 @@ void btrfs_mark_bg_unused(struct btrfs_block_group *bg)
        spin_unlock(&fs_info->unused_bgs_lock);
  }

+void btrfs_reclaim_bgs(struct btrfs_fs_info *fs_info)

+{
+       struct btrfs_block_group *bg;
+       struct btrfs_space_info *space_info;
+       int ret = 0;
+
+       if (!test_bit(BTRFS_FS_OPEN, &fs_info->flags))
+               return;
+
+       if (!btrfs_exclop_start(fs_info, BTRFS_EXCLOP_BALANCE))
+               return;
+
+       mutex_lock(&fs_info->reclaim_bgs_lock);
+       while (!list_empty(&fs_info->reclaim_bgs)) {
+               bg = list_first_entry(&fs_info->reclaim_bgs,
+                                     struct btrfs_block_group,
+                                     bg_list);
+               list_del_init(&bg->bg_list);
+
+               space_info = bg->space_info;
+               mutex_unlock(&fs_info->reclaim_bgs_lock);
+
+               /* Don't want to race with allocators so take the groups_sem */
+               down_write(&space_info->groups_sem);
+
+               spin_lock(&bg->lock);
+               if (bg->reserved || bg->pinned || bg->ro) {
+                       /*
+                        * We want to bail if we made new allocations or have
+                        * outstanding allocations in this block group.  We do
+                        * the ro check in case balance is currently acting on
+                        * this block group.
+                        */
+                       spin_unlock(&bg->lock);
+                       up_write(&space_info->groups_sem);
+                       goto next;
+               }
+               spin_unlock(&bg->lock);
+
+               ret = inc_block_group_ro(bg, 0);
+               up_write(&space_info->groups_sem);
+               if (ret < 0) {
+                       ret = 0;
+                       goto next;
+               }
+
+               btrfs_info(fs_info, "reclaiming chunk %llu", bg->start);
+               trace_btrfs_reclaim_block_group(bg);
+               ret = btrfs_relocate_chunk(fs_info, bg->start);
+               if (ret)
+                       btrfs_err(fs_info, "error relocating chunk %llu",
+                                 bg->start);
+
+next:
+               btrfs_put_block_group(bg);
+               mutex_lock(&fs_info->reclaim_bgs_lock);
+       }
+       mutex_unlock(&fs_info->reclaim_bgs_lock);
+       btrfs_exclop_finish(fs_info);
+}
+
+void btrfs_mark_bg_to_reclaim(struct btrfs_block_group *bg)
+{
+       struct btrfs_fs_info *fs_info = bg->fs_info;
+
+       mutex_lock(&fs_info->reclaim_bgs_lock);
+       if (list_empty(&bg->bg_list)) {
+               btrfs_get_block_group(bg);
+               trace_btrfs_add_reclaim_block_group(bg);
+               list_add_tail(&bg->bg_list, &fs_info->reclaim_bgs);
+       }
+       mutex_unlock(&fs_info->reclaim_bgs_lock);
+}
+
  static int read_bg_from_eb(struct btrfs_fs_info *fs_info, struct btrfs_key 
*key,
                           struct btrfs_path *path)
  {
@@ -3390,6 +3464,16 @@ int btrfs_free_block_groups(struct btrfs_fs_info *info)
        }
        spin_unlock(&info->unused_bgs_lock);

+ mutex_lock(&info->reclaim_bgs_lock);

+       while (!list_empty(&info->reclaim_bgs)) {
+               block_group = list_first_entry(&info->reclaim_bgs,
+                                              struct btrfs_block_group,
+                                              bg_list);
+               list_del_init(&block_group->bg_list);
+               btrfs_put_block_group(block_group);
+       }
+       mutex_unlock(&info->reclaim_bgs_lock);
+
        spin_lock(&info->block_group_cache_lock);
        while ((n = rb_last(&info->block_group_cache_tree)) != NULL) {
                block_group = rb_entry(n, struct btrfs_block_group,
diff --git a/fs/btrfs/block-group.h b/fs/btrfs/block-group.h
index 3ecc3372a5ce..e75c79676241 100644
--- a/fs/btrfs/block-group.h
+++ b/fs/btrfs/block-group.h
@@ -264,6 +264,8 @@ int btrfs_remove_block_group(struct btrfs_trans_handle 
*trans,
                             u64 group_start, struct extent_map *em);
  void btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info);
  void btrfs_mark_bg_unused(struct btrfs_block_group *bg);
+void btrfs_reclaim_bgs(struct btrfs_fs_info *fs_info);
+void btrfs_mark_bg_to_reclaim(struct btrfs_block_group *bg);
  int btrfs_read_block_groups(struct btrfs_fs_info *info);
  int btrfs_make_block_group(struct btrfs_trans_handle *trans, u64 bytes_used,
                           u64 type, u64 chunk_offset, u64 size);
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 34ec82d6df3e..0b438b97fed4 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -938,6 +938,7 @@ struct btrfs_fs_info {
        struct list_head unused_bgs;
        struct mutex unused_bg_unpin_mutex;
        struct mutex reclaim_bgs_lock;
+       struct list_head reclaim_bgs;

/* Cached block sizes */

        u32 nodesize;
@@ -978,6 +979,8 @@ struct btrfs_fs_info {
        spinlock_t treelog_bg_lock;
        u64 treelog_bg;

+ int bg_reclaim_threshold;

+
  #ifdef CONFIG_BTRFS_FS_REF_VERIFY
        spinlock_t ref_verify_lock;
        struct rb_root block_tree;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index f9250f14fc1e..d4fccf113df1 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1815,6 +1815,13 @@ static int cleaner_kthread(void *arg)
                 * unused block groups.
                 */
                btrfs_delete_unused_bgs(fs_info);
+
+               /*
+                * Reclaim block groups in the reclaim_bgs list after we deleted
+                * all unused block_groups. This possibly gives us some more 
free
+                * space.
+                */
+               btrfs_reclaim_bgs(fs_info);
  sleep:
                clear_and_wake_up_bit(BTRFS_FS_CLEANER_RUNNING, 
&fs_info->flags);
                if (kthread_should_park())
@@ -2797,12 +2804,14 @@ void btrfs_init_fs_info(struct btrfs_fs_info *fs_info)
        mutex_init(&fs_info->reloc_mutex);
        mutex_init(&fs_info->delalloc_root_mutex);
        mutex_init(&fs_info->zoned_meta_io_lock);
+       mutex_init(&fs_info->reclaim_bgs_lock);
        seqlock_init(&fs_info->profiles_lock);

INIT_LIST_HEAD(&fs_info->dirty_cowonly_roots);

        INIT_LIST_HEAD(&fs_info->space_info);
        INIT_LIST_HEAD(&fs_info->tree_mod_seq_list);
        INIT_LIST_HEAD(&fs_info->unused_bgs);
+       INIT_LIST_HEAD(&fs_info->reclaim_bgs);
  #ifdef CONFIG_BTRFS_DEBUG
        INIT_LIST_HEAD(&fs_info->allocated_roots);
        INIT_LIST_HEAD(&fs_info->allocated_ebs);
@@ -2891,6 +2900,8 @@ void btrfs_init_fs_info(struct btrfs_fs_info *fs_info)
        fs_info->swapfile_pins = RB_ROOT;

fs_info->send_in_progress = 0;

+
+       fs_info->bg_reclaim_threshold = 75;
  }

static int init_mount_fs_info(struct btrfs_fs_info *fs_info, struct super_block *sb)

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index 9988decd5717..e54466fc101f 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -11,6 +11,7 @@
  #include <linux/ratelimit.h>
  #include <linux/error-injection.h>
  #include <linux/sched/mm.h>
+#include "misc.h"
  #include "ctree.h"
  #include "free-space-cache.h"
  #include "transaction.h"
@@ -2539,6 +2540,7 @@ int __btrfs_add_free_space(struct btrfs_fs_info *fs_info,
  static int __btrfs_add_free_space_zoned(struct btrfs_block_group *block_group,
                                        u64 bytenr, u64 size, bool used)
  {
+       struct btrfs_fs_info *fs_info = block_group->fs_info;
        struct btrfs_free_space_ctl *ctl = block_group->free_space_ctl;
        u64 offset = bytenr - block_group->start;
        u64 to_free, to_unusable;
@@ -2569,8 +2571,13 @@ static int __btrfs_add_free_space_zoned(struct 
btrfs_block_group *block_group,
        }

/* All the region is now unusable. Mark it as unused and reclaim */

-       if (block_group->zone_unusable == block_group->length)
+       if (block_group->zone_unusable == block_group->length) {
                btrfs_mark_bg_unused(block_group);
+       } else if (block_group->zone_unusable >=
+                  div_factor_fine(block_group->length,
+                                  fs_info->bg_reclaim_threshold)) {
+               btrfs_mark_bg_to_reclaim(block_group);
+       }

return 0;

  }
diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 6eb1c50fa98c..bf38c7c6b804 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -965,6 +965,40 @@ static ssize_t btrfs_read_policy_store(struct kobject 
*kobj,
  }
  BTRFS_ATTR_RW(, read_policy, btrfs_read_policy_show, btrfs_read_policy_store);

+static ssize_t btrfs_bg_reclaim_threshold_show(struct kobject *kobj,

+                                              struct kobj_attribute *a,
+                                              char *buf)
+{
+       struct btrfs_fs_info *fs_info = to_fs_info(kobj);
+       ssize_t ret;
+
+       ret = scnprintf(buf, PAGE_SIZE, "%d\n", fs_info->bg_reclaim_threshold);
+
+       return ret;
+}
+
+static ssize_t btrfs_bg_reclaim_threshold_store(struct kobject *kobj,
+                                               struct kobj_attribute *a,
+                                               const char *buf, size_t len)
+{
+       struct btrfs_fs_info *fs_info = to_fs_info(kobj);
+       int thresh;
+       int ret;
+
+       ret = kstrtoint(buf, 10, &thresh);
+       if (ret)
+               return ret;
+
+       if (thresh <= 50 || thresh > 100)
+               return -EINVAL;
+
+       fs_info->bg_reclaim_threshold = thresh;
+
+       return len;
+}
+BTRFS_ATTR_RW(, bg_reclaim_threshold, btrfs_bg_reclaim_threshold_show,
+             btrfs_bg_reclaim_threshold_store);
+
  static const struct attribute *btrfs_attrs[] = {
        BTRFS_ATTR_PTR(, label),
        BTRFS_ATTR_PTR(, nodesize),
@@ -976,6 +1010,7 @@ static const struct attribute *btrfs_attrs[] = {
        BTRFS_ATTR_PTR(, exclusive_operation),
        BTRFS_ATTR_PTR(, generation),
        BTRFS_ATTR_PTR(, read_policy),
+       BTRFS_ATTR_PTR(, bg_reclaim_threshold),
        NULL,
  };

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c

index fb785ff53a27..c78b5ce49d47 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -3098,7 +3098,7 @@ int btrfs_remove_chunk(struct btrfs_trans_handle *trans, 
u64 chunk_offset)
        return ret;
  }

-static int btrfs_relocate_chunk(struct btrfs_fs_info *fs_info, u64 chunk_offset)

+int btrfs_relocate_chunk(struct btrfs_fs_info *fs_info, u64 chunk_offset)
  {
        struct btrfs_root *root = fs_info->chunk_root;
        struct btrfs_trans_handle *trans;
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index d4c3e0dd32b8..9c0d84e5ec06 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -484,6 +484,7 @@ void btrfs_describe_block_groups(u64 flags, char *buf, u32 
size_buf);
  int btrfs_resume_balance_async(struct btrfs_fs_info *fs_info);
  int btrfs_recover_balance(struct btrfs_fs_info *fs_info);
  int btrfs_pause_balance(struct btrfs_fs_info *fs_info);
+int btrfs_relocate_chunk(struct btrfs_fs_info *fs_info, u64 chunk_offset);
  int btrfs_cancel_balance(struct btrfs_fs_info *fs_info);
  int btrfs_create_uuid_tree(struct btrfs_fs_info *fs_info);
  int btrfs_uuid_scan_kthread(void *data);
diff --git a/include/trace/events/btrfs.h b/include/trace/events/btrfs.h
index 0551ea65374f..a41dd8a0c730 100644
--- a/include/trace/events/btrfs.h
+++ b/include/trace/events/btrfs.h
@@ -1903,6 +1903,18 @@ DEFINE_EVENT(btrfs__block_group, 
btrfs_add_unused_block_group,
        TP_ARGS(bg_cache)
  );

+DEFINE_EVENT(btrfs__block_group, btrfs_add_reclaim_block_group,

+       TP_PROTO(const struct btrfs_block_group *bg_cache),
+
+       TP_ARGS(bg_cache)
+);
+
+DEFINE_EVENT(btrfs__block_group, btrfs_reclaim_block_group,
+       TP_PROTO(const struct btrfs_block_group *bg_cache),
+
+       TP_ARGS(bg_cache)
+);
+
  DEFINE_EVENT(btrfs__block_group, btrfs_skip_unused_block_group,
        TP_PROTO(const struct btrfs_block_group *bg_cache),