On Tue, Apr 13, 2021 at 6:48 PM Johannes Thumshirn <johannes.thumsh...@wdc.com> wrote: > > On 13/04/2021 14:57, Filipe Manana wrote: > > And what about the other mechanism that triggers discards on pinned > > extents, after the transaction commits the super blocks? > > Why isn't that happening (with -o discard=sync)? We create the delayed > > references to drop extents from the relocated block group, which > > results in pinning extents. > > This is the case that surprised me that it isn't working for you. > > I think this is the case. I would have expected to end up in this > part of btrfs_finish_extent_commit(): > > > /* > * Transaction is finished. We don't need the lock anymore. We > * do need to clean up the block groups in case of a transaction > * abort. > */ > deleted_bgs = &trans->transaction->deleted_bgs; > list_for_each_entry_safe(block_group, tmp, deleted_bgs, bg_list) { > u64 trimmed = 0; > > ret = -EROFS; > if (!TRANS_ABORTED(trans)) > ret = btrfs_discard_extent(fs_info, > block_group->start, > block_group->length, > &trimmed); > > list_del_init(&block_group->bg_list); > btrfs_unfreeze_block_group(block_group); > btrfs_put_block_group(block_group); > > if (ret) { > const char *errstr = btrfs_decode_error(ret); > btrfs_warn(fs_info, > "discard failed while removing blockgroup: > errno=%d %s", > ret, errstr); > } > } > > and the btrfs_discard_extent() over the whole block group would then trigger a > REQ_OP_ZONE_RESET operation, resetting the device's zone. > > But as btrfs_delete_unused_bgs() doesn't add the block group to the > ->deleted_bgs list, we're not reaching above code. I /think/ (i.e. > verification > pending) the -o discard=sync case works for regular block devices, as each > extent > is discarded on it's own, by this (also in btrfs_finish_extent_commit()): > > while (!TRANS_ABORTED(trans)) { > struct extent_state *cached_state = NULL; > > mutex_lock(&fs_info->unused_bg_unpin_mutex); > ret = find_first_extent_bit(unpin, 0, &start, &end, > EXTENT_DIRTY, &cached_state); > if (ret) { > mutex_unlock(&fs_info->unused_bg_unpin_mutex); > break; > } > > if (btrfs_test_opt(fs_info, DISCARD_SYNC)) > ret = btrfs_discard_extent(fs_info, start, > end + 1 - start, NULL); > > clear_extent_dirty(unpin, start, end, &cached_state); > unpin_extent_range(fs_info, start, end, true); > mutex_unlock(&fs_info->unused_bg_unpin_mutex); > free_extent_state(cached_state); > cond_resched(); > } > > If this is the case, my patch will essentially discard the data twice, for a > non-zoned block device, which is certainly not ideal.
Yep, that's what puzzled me, why the need to do it for non-zoned file systems when using -o discard=sync. I assumed you ran into a case where discard was not happening due to some bug bug in the extent pinning/unpinning mechanism. > So the correct fix would > be to get the block group into the 'trans->transaction->deleted_bgs' list > after relocation, which would work if we wouldn't check for block_group->ro in > btrfs_delete_unused_bgs(), but I suppose this check is there for a reason. Actually the check for ->ro does not make sense anymore since I introduced the delete_unused_bgs_mutex in commit 67c5e7d464bc466471b05e027abe8a6b29687ebd. When the ->ro check was added (47ab2a6c689913db23ccae38349714edf8365e0a), it was meant to prevent the cleaner kthread and relocation tasks from calling btrfs_remove_chunk() concurrently, but checking for ->ro only was buggy, hence the addition of delete_unused_bgs_mutex later. > > How about changing the patch to the following: Looks good. However would just removing the ->ro check by enough as well? Thanks Johannes. > > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c > index 6d9b2369f17a..ba13b2ea3c6f 100644 > --- a/fs/btrfs/volumes.c > +++ b/fs/btrfs/volumes.c > @@ -3103,6 +3103,9 @@ static 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; > struct btrfs_block_group *block_group; > + u64 length; > int ret; > > /* > @@ -3130,8 +3133,16 @@ static int btrfs_relocate_chunk(struct btrfs_fs_info > *fs_info, u64 chunk_offset) > if (!block_group) > return -ENOENT; > btrfs_discard_cancel_work(&fs_info->discard_ctl, block_group); > + length = block_group->length; > btrfs_put_block_group(block_group); > > + /* > + * For a zoned filesystem we need to discard/zone-reset here, as the > + * discard code won't discard the whole block-group, but only single > + * extents. > + */ > + if (btrfs_is_zoned(fs_info)) { > + ret = btrfs_discard_extent(fs_info, chunk_offset, length, > NULL); > + if (ret) /* Non working discard is not fatal */ > + btrfs_warn(fs_info, "discarding chunk %llu failed", > + chunk_offset); > + } > + > trans = btrfs_start_trans_remove_block_group(root->fs_info, > chunk_offset); > if (IS_ERR(trans)) { -- Filipe David Manana, “Whether you think you can, or you think you can't — you're right.”