@@ -3409,7 +3409,7 @@ 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
@@ -3435,7 +3435,14 @@ 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);
@@ -3519,7 +3526,15 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags)
* 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