diff options
Diffstat (limited to 'fs/btrfs')
| -rw-r--r-- | fs/btrfs/space-info.c | 47 |
1 files changed, 47 insertions, 0 deletions
diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c index bbddb8b5eff1..71aa9e0de61e 100644 --- a/fs/btrfs/space-info.c +++ b/fs/btrfs/space-info.c @@ -998,6 +998,53 @@ static void btrfs_async_reclaim_metadata_space(struct work_struct *work) } while (flush_state <= COMMIT_TRANS); } +/* + * FLUSH_DELALLOC_WAIT: + * Space is freed from flushing delalloc in one of two ways. + * + * 1) compression is on and we allocate less space than we reserved + * 2) we are overwriting existing space + * + * For #1 that extra space is reclaimed as soon as the delalloc pages are + * COWed, by way of btrfs_add_reserved_bytes() which adds the actual extent + * length to ->bytes_reserved, and subtracts the reserved space from + * ->bytes_may_use. + * + * For #2 this is trickier. Once the ordered extent runs we will drop the + * extent in the range we are overwriting, which creates a delayed ref for + * that freed extent. This however is not reclaimed until the transaction + * commits, thus the next stages. + * + * RUN_DELAYED_IPUTS + * If we are freeing inodes, we want to make sure all delayed iputs have + * completed, because they could have been on an inode with i_nlink == 0, and + * thus have been truncated and freed up space. But again this space is not + * immediately re-usable, it comes in the form of a delayed ref, which must be + * run and then the transaction must be committed. + * + * FLUSH_DELAYED_REFS + * The above two cases generate delayed refs that will affect + * ->total_bytes_pinned. However this counter can be inconsistent with + * reality if there are outstanding delayed refs. This is because we adjust + * the counter based solely on the current set of delayed refs and disregard + * any on-disk state which might include more refs. So for example, if we + * have an extent with 2 references, but we only drop 1, we'll see that there + * is a negative delayed ref count for the extent and assume that the space + * will be freed, and thus increase ->total_bytes_pinned. + * + * Running the delayed refs gives us the actual real view of what will be + * freed at the transaction commit time. This stage will not actually free + * space for us, it just makes sure that may_commit_transaction() has all of + * the information it needs to make the right decision. + * + * COMMIT_TRANS + * This is where we reclaim all of the pinned space generated by the previous + * two stages. We will not commit the transaction if we don't think we're + * likely to satisfy our request, which means if our current free space + + * total_bytes_pinned < reservation we will not commit. This is why the + * previous states are actually important, to make sure we know for sure + * whether committing the transaction will allow us to make progress. + */ static const enum btrfs_flush_state data_flush_states[] = { FLUSH_DELALLOC_WAIT, RUN_DELAYED_IPUTS, |