• Dennis Zhou's avatar
    btrfs: handle empty block_group removal for async discard · 6e80d4f8
    Dennis Zhou authored
    block_group removal is a little tricky. It can race with the extent
    allocator, the cleaner thread, and balancing. The current path is for a
    block_group to be added to the unused_bgs list. Then, when the cleaner
    thread comes around, it starts a transaction and then proceeds with
    removing the block_group. Extents that are pinned are subsequently
    removed from the pinned trees and then eventually a discard is issued
    for the entire block_group.
    
    Async discard introduces another player into the game, the discard
    workqueue. While it has none of the racing issues, the new problem is
    ensuring we don't leave free space untrimmed prior to forgetting the
    block_group.  This is handled by placing fully free block_groups on a
    separate discard queue. This is necessary to maintain discarding order
    as in the future we will slowly trim even fully free block_groups. The
    ordering helps us make progress on the same block_group rather than say
    the last fully freed block_group or needing to search through the fully
    freed block groups at the beginning of a list and insert after.
    
    The new order of events is a fully freed block group gets placed on the
    unused discard queue first. Once it's processed, it will be placed on
    the unusued_bgs list and then the original sequence of events will
    happen, just without the final whole block_group discard.
    
    The mount flags can change when processing unused_bgs, so when flipping
    from DISCARD to DISCARD_ASYNC, the unused_bgs must be punted to the
    discard_list to be trimmed. If we flip off DISCARD_ASYNC, we punt
    free block groups on the discard_list to the unused_bg queue which will
    do the final discard for us.
    Reviewed-by: default avatarJosef Bacik <josef@toxicpanda.com>
    Signed-off-by: default avatarDennis Zhou <dennis@kernel.org>
    Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
    Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
    6e80d4f8
discard.h 923 Bytes