This function is the entry to the extent allocator and as such has
quite a number of parameters. Some of those have subtle effects on the
allocation algorithm. Document the parameters.

Signed-off-by: Nikolay Borisov <nbori...@suse.com>
---
 fs/btrfs/extent-tree.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index bb1861033071..60ece25101ee 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -7961,6 +7961,51 @@ static void dump_space_info(struct btrfs_fs_info 
*fs_info,
        up_read(&info->groups_sem);
 }
 
+/*
+ * btrfs_reserve_extent - entry point to the extent allocator. Tries to find a
+ *                       hole that is at least as big as @num_bytes.
+ *
+ * @root           -   The root that will contains this extent
+ *
+ * @ram_bytes      -   The amount of space in ram that @num_bytes take. This
+ *                     is used for accounting purposes. This value differs
+ *                     than @num_bytes only in the case of compressed extents.
+ *
+ * @num_bytes      -   Number of bytes to allocates on-disk.
+ *
+ * @min_alloc_size -   Indicates the minimum amount of memory that the
+ *                     allocator should try to satisfy. In some cases
+ *                     @num_bytes may be larger than what is required and if
+ *                     the filesystem is fragmented then allocation fails.
+ *                     However, the presence of @min_alloc_size gives a
+ *                     chance to try and satisfy the smaller allocation.
+ *
+ * @empty_size     -   A hint that you plan on doing more cow. This is the
+ *                     size in bytes the allocator should try to find free
+ *                     next to the block it returns.  This is just a hint and
+ *                     may be ignored by the allocator.
+ *
+ * @hint_byte      -   Hint to the allocator to start searching above the byte
+ *                     address passed. It might be ignored.
+ *
+ * @ins            -   This key is modified to record the found hole. It will
+ *                     have the following values:
+ *                     ins->objectid == start position
+ *                     ins->flags = BTRFS_EXTENT_ITEM_KEY
+ *                     ins->offset == the size of the hole.
+ *
+ * @is_data        -   Boolean flag indicating whether an extent is
+ *                     allocated for data (true) or metadata (false)
+ *
+ * @delalloc       -   Boolean flag indicating whether this allocation is for
+ *                     delalloc or not. If 'true' data_rwsem of block groups
+ *                     is going to be acquired.
+ *
+ *
+ * Returns 0 when an allocation succeeded or < 0 when an error occurred. In
+ * case -ENOSPC is returned then @ins->offset will contain the size of the
+ * largest available hole the allocator managed to find.
+ */
 int btrfs_reserve_extent(struct btrfs_root *root, u64 ram_bytes,
                         u64 num_bytes, u64 min_alloc_size,
                         u64 empty_size, u64 hint_byte,
-- 
2.7.4

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