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. 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.
How about changing the patch to the following:
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)) {
