diff options
| -rw-r--r-- | fs/btrfs/block-group.c | 21 |
1 files changed, 18 insertions, 3 deletions
diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c index f971d043469c..444e9c89ff3e 100644 --- a/fs/btrfs/block-group.c +++ b/fs/btrfs/block-group.c @@ -3429,7 +3429,7 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags) /* * Normally we are not expected to fail with -ENOSPC here, since we have * previously reserved space in the system space_info and allocated one - * new system chunk if necessary. However there are two exceptions: + * new system chunk if necessary. However there are three exceptions: * * 1) We may have enough free space in the system space_info but all the * existing system block groups have a profile which can not be used @@ -3455,7 +3455,14 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags) * with enough free space got turned into RO mode by a running scrub, * and in this case we have to allocate a new one and retry. We only * need do this allocate and retry once, since we have a transaction - * handle and scrub uses the commit root to search for block groups. + * handle and scrub uses the commit root to search for block groups; + * + * 3) We had one system block group with enough free space when we called + * check_system_chunk(), but after that, right before we tried to + * allocate the last extent buffer we needed, a discard operation came + * in and it temporarily removed the last free space entry from the + * block group (discard removes a free space entry, discards it, and + * then adds back the entry to the block group cache). */ if (ret == -ENOSPC) { const u64 sys_flags = btrfs_system_alloc_profile(trans->fs_info); @@ -3539,7 +3546,15 @@ out: * properly, either intentionally or as a bug. One example where this is * done intentionally is fsync, as it does not reserve any transaction units * and ends up allocating a variable number of metadata extents for log - * tree extent buffers. + * tree extent buffers; + * + * 4) The task has reserved enough transaction units / metadata space, but right + * before it tries to allocate the last extent buffer it needs, a discard + * operation comes in and, temporarily, removes the last free space entry from + * the only metadata block group that had free space (discard starts by + * removing a free space entry from a block group, then does the discard + * operation and, once it's done, it adds back the free space entry to the + * block group). * * We also need this 2 phases setup when adding a device to a filesystem with * a seed device - we must create new metadata and system chunks without adding |