Re: [PATCH v2] btrfs: iterate over unused chunk space in FITRIM

[Filipe David Manana <fdmanana@gmail.com>]

On Tue, May 5, 2015 at 7:19 PM, Jeff Mahoney <jeffm@suse.com> wrote:
> Since we now clean up block groups automatically as they become
> empty, iterating over block groups is no longer sufficient to discard
> unused space.
>
> This patch iterates over the unused chunk space and discards any regions
> that are unallocated, regardless of whether they were ever used.  This is
> a change for btrfs but is consistent with other file systems.
>
> We do this in a transactionless manner since the discard process can take
> a substantial amount of time and a transaction would need to be started
> before the acquisition of the device list lock.  That would mean a
> transaction would be held open across /all/ of the discards collectively.
> In order to prevent other threads from allocating or freeing chunks, we
> hold the chunks lock across the search and discard calls.  We release it
> between searches to allow the file system to perform more-or-less
> normally.  Since the running transaction can commit and disappear while
> we're using the transaction pointer, we take a reference to it and
> release it after the search.  This is safe since it would happen normally
> at the end of the transaction commit after any locks are released anyway.
>
> Signed-off-by: Jeff Mahoney <jeffm@suse.com>
> ---

Hi Jeff,

Missing changelog describing what changed between v1 and v2.

>  fs/btrfs/extent-tree.c | 96 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/btrfs/volumes.c     | 61 ++++++++++++++++++++------------
>  fs/btrfs/volumes.h     |  3 ++
>  3 files changed, 137 insertions(+), 23 deletions(-)
>
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 0ec8e22..6d1d74d 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -10031,10 +10031,94 @@ int btrfs_error_unpin_extent_range(struct btrfs_root *root, u64 start, u64 end)
>         return unpin_extent_range(root, start, end, false);
>  }
>
> +/*
> + * It used to be that old block groups would be left around forever.
> + * Iterating over them would be enough to trim unused space.  Since we
> + * now automatically remove them, we also need to iterate over unallocated
> + * space.
> + *
> + * We don't want a transaction for this since the discard may take a
> + * substantial amount of time.  We don't require that a transaction be
> + * running, but we do need to take a running transaction into account
> + * to ensure that we're not discarding chunks that were released in
> + * the current transaction.
> + *
> + * Holding the chunks lock will prevent other threads from allocating
> + * or releasing chunks, but it won't prevent a running transaction
> + * from committing and releasing the memory that the pending chunks
> + * list head uses.  For that, we need to take a reference to the
> + * transaction.
> + */
> +static int btrfs_trim_free_extents(struct btrfs_device *device,
> +                                  u64 minlen, u64 *trimmed)
> +{
> +       u64 start = 0, len = 0;
> +       int ret;
> +
> +       *trimmed = 0;
> +
> +       /* Not writeable = nothing to do. */
> +       if (!device->writeable)
> +               return 0;
> +
> +       /* No free space = nothing to do. */
> +       if (device->total_bytes <= device->bytes_used)
> +               return 0;
> +
> +       ret = 0;
> +
> +       while (1) {
> +               struct btrfs_fs_info *fs_info = device->dev_root->fs_info;
> +               struct btrfs_transaction *trans;
> +
> +               ret = mutex_lock_interruptible(&fs_info->chunk_mutex);
> +               if (ret)
> +                       return ret;
> +
> +               spin_lock(&fs_info->trans_lock);
> +               trans = fs_info->running_transaction;
> +               if (trans)
> +                       atomic_inc(&trans->use_count);
> +               spin_unlock(&fs_info->trans_lock);
> +
> +               ret = find_free_dev_extent_start(trans, device, minlen, start,
> +                                                &start, &len);
> +               if (trans)
> +                       btrfs_put_transaction(trans);
> +
> +               if (ret) {
> +                       mutex_unlock(&fs_info->chunk_mutex);
> +                       if (ret == -ENOSPC)
> +                               ret = 0;
> +                       break;
> +               }
> +
> +               ret = btrfs_issue_discard(device->bdev, start, len);
> +               mutex_unlock(&fs_info->chunk_mutex);
> +
> +               if (ret)
> +                       break;
> +
> +               start += len;
> +               *trimmed += len;
> +
> +               if (fatal_signal_pending(current)) {
> +                       ret = -ERESTARTSYS;
> +                       break;
> +               }
> +
> +               cond_resched();
> +       }
> +
> +       return ret;
> +}
> +
>  int btrfs_trim_fs(struct btrfs_root *root, struct fstrim_range *range)
>  {
>         struct btrfs_fs_info *fs_info = root->fs_info;
>         struct btrfs_block_group_cache *cache = NULL;
> +       struct btrfs_device *device;
> +       struct list_head *devices;
>         u64 group_trimmed;
>         u64 start;
>         u64 end;
> @@ -10089,6 +10173,18 @@ int btrfs_trim_fs(struct btrfs_root *root, struct fstrim_range *range)
>                 cache = next_block_group(fs_info->tree_root, cache);
>         }
>
> +       mutex_lock(&root->fs_info->fs_devices->device_list_mutex);
> +       devices = &root->fs_info->fs_devices->alloc_list;
> +       list_for_each_entry(device, devices, dev_alloc_list) {
> +               ret = btrfs_trim_free_extents(device, range->minlen,
> +                                             &group_trimmed);
> +               if (ret)
> +                       break;
> +
> +               trimmed += group_trimmed;
> +       }
> +       mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
> +
>         range->len = trimmed;
>         return ret;
>  }
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 96aebf3..ada8965 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -1051,15 +1051,19 @@ out:
>         return ret;
>  }
>
> -static int contains_pending_extent(struct btrfs_trans_handle *trans,
> +static int contains_pending_extent(struct btrfs_transaction *transaction,
>                                    struct btrfs_device *device,
>                                    u64 *start, u64 len)
>  {
> +       struct btrfs_fs_info *fs_info = device->dev_root->fs_info;
>         struct extent_map *em;
> -       struct list_head *search_list = &trans->transaction->pending_chunks;
> +       struct list_head *search_list = &transaction->pending_chunks;

transaction can be NULL so we need to check for that first (as you do below).

>         int ret = 0;
>         u64 physical_start = *start;
>
> +       if (!transaction)
> +               search_list = &fs_info->pinned_chunks;
> +
>  again:
>         list_for_each_entry(em, search_list, list) {
>                 struct map_lookup *map;
> @@ -1078,8 +1082,8 @@ again:
>                         ret = 1;
>                 }
>         }
> -       if (search_list == &trans->transaction->pending_chunks) {
> -               search_list = &trans->root->fs_info->pinned_chunks;
> +       if (search_list == &transaction->pending_chunks) {
> +               search_list = &fs_info->pinned_chunks;
>                 goto again;
>         }
>
> @@ -1088,12 +1092,13 @@ again:
>
>
>  /*
> - * find_free_dev_extent - find free space in the specified device
> - * @device:    the device which we search the free space in
> - * @num_bytes: the size of the free space that we need
> - * @start:     store the start of the free space.
> - * @len:       the size of the free space. that we find, or the size of the max
> - *             free space if we don't find suitable free space
> + * find_free_dev_extent_start - find free space in the specified device
> + * @device:      the device which we search the free space in
> + * @num_bytes:   the size of the free space that we need
> + * @search_start: the position from which to begin the search
> + * @start:       store the start of the free space.
> + * @len:         the size of the free space. that we find, or the size
> + *               of the max free space if we don't find suitable free space
>   *
>   * this uses a pretty simple search, the expectation is that it is
>   * called very infrequently and that a given device has a small number
> @@ -1107,9 +1112,9 @@ again:
>   * But if we don't find suitable free space, it is used to store the size of
>   * the max free space.
>   */
> -int find_free_dev_extent(struct btrfs_trans_handle *trans,
> -                        struct btrfs_device *device, u64 num_bytes,
> -                        u64 *start, u64 *len)
> +int find_free_dev_extent_start(struct btrfs_transaction *transaction,
> +                              struct btrfs_device *device, u64 num_bytes,
> +                              u64 search_start, u64 *start, u64 *len)
>  {
>         struct btrfs_key key;
>         struct btrfs_root *root = device->dev_root;
> @@ -1119,19 +1124,11 @@ int find_free_dev_extent(struct btrfs_trans_handle *trans,
>         u64 max_hole_start;
>         u64 max_hole_size;
>         u64 extent_end;
> -       u64 search_start;
>         u64 search_end = device->total_bytes;
>         int ret;
>         int slot;
>         struct extent_buffer *l;
>
> -       /* FIXME use last free of some kind */
> -
> -       /* we don't want to overwrite the superblock on the drive,
> -        * so we make sure to start at an offset of at least 1MB
> -        */
> -       search_start = max(root->fs_info->alloc_start, 1024ull * 1024);
> -
>         path = btrfs_alloc_path();
>         if (!path)
>                 return -ENOMEM;
> @@ -1192,7 +1189,7 @@ again:
>                          * Have to check before we set max_hole_start, otherwise
>                          * we could end up sending back this offset anyway.
>                          */
> -                       if (contains_pending_extent(trans, device,
> +                       if (contains_pending_extent(transaction, device,
>                                                     &search_start,
>                                                     hole_size)) {
>                                 if (key.offset >= search_start) {
> @@ -1241,7 +1238,7 @@ next:
>         if (search_end > search_start) {
>                 hole_size = search_end - search_start;
>
> -               if (contains_pending_extent(trans, device, &search_start,
> +               if (contains_pending_extent(transaction, device, &search_start,
>                                             hole_size)) {
>                         btrfs_release_path(path);
>                         goto again;
> @@ -1267,6 +1264,24 @@ out:
>         return ret;
>  }
>
> +int find_free_dev_extent(struct btrfs_trans_handle *trans,
> +                        struct btrfs_device *device, u64 num_bytes,
> +                        u64 *start, u64 *len)
> +{
> +       struct btrfs_root *root = device->dev_root;
> +       u64 search_start;
> +
> +       /* FIXME use last free of some kind */
> +
> +       /*
> +        * we don't want to overwrite the superblock on the drive,
> +        * so we make sure to start at an offset of at least 1MB
> +        */
> +       search_start = max(root->fs_info->alloc_start, 1024ull * 1024);
> +       return find_free_dev_extent_start(trans->transaction, device,
> +                                         num_bytes, search_start, start, len);
> +}
> +
>  static int btrfs_free_dev_extent(struct btrfs_trans_handle *trans,
>                           struct btrfs_device *device,
>                           u64 start, u64 *dev_extent_len)
> diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
> index ebc3133..30918a8 100644
> --- a/fs/btrfs/volumes.h
> +++ b/fs/btrfs/volumes.h
> @@ -449,6 +449,9 @@ int btrfs_cancel_balance(struct btrfs_fs_info *fs_info);
>  int btrfs_create_uuid_tree(struct btrfs_fs_info *fs_info);
>  int btrfs_check_uuid_tree(struct btrfs_fs_info *fs_info);
>  int btrfs_chunk_readonly(struct btrfs_root *root, u64 chunk_offset);
> +int find_free_dev_extent_start(struct btrfs_transaction *transaction,
> +                        struct btrfs_device *device, u64 num_bytes,
> +                        u64 search_start, u64 *start, u64 *max_avail);
>  int find_free_dev_extent(struct btrfs_trans_handle *trans,
>                          struct btrfs_device *device, u64 num_bytes,
>                          u64 *start, u64 *max_avail);
> --
> 1.8.5.6
>
>
> --
> Jeff Mahoney
> SUSE Labs
> --
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-- 
Filipe David Manana,

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."