nilfs2: improve kernel-doc comments for b-tree node helpers

Revise kernel-doc comments for helper functions related to changing the
search key for b-tree node blocks, and eliminate the following warnings
output by the kernel-doc script:

 fs/nilfs2/btnode.c:175: warning: Function parameter or struct member 'btnc'
  not described in 'nilfs_btnode_prepare_change_key'
 fs/nilfs2/btnode.c:175: warning: Function parameter or struct member 'ctxt'
  not described in 'nilfs_btnode_prepare_change_key'
 fs/nilfs2/btnode.c:238: warning: Function parameter or struct member 'btnc'
  not described in 'nilfs_btnode_commit_change_key'
 fs/nilfs2/btnode.c:238: warning: Function parameter or struct member 'ctxt'
  not described in 'nilfs_btnode_commit_change_key'
 fs/nilfs2/btnode.c:278: warning: Function parameter or struct member 'btnc'
  not described in 'nilfs_btnode_abort_change_key'
 fs/nilfs2/btnode.c:278: warning: Function parameter or struct member 'ctxt'
  not described in 'nilfs_btnode_abort_change_key'

Link: https://lkml.kernel.org/r/20240816074319.3253-4-konishi.ryusuke@gmail.comSigned-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/nilfs2/btnode.c

@@ -179,11 +179,32 @@ void nilfs_btnode_delete(struct buffer_head *bh)
 }
 
 /**
- * nilfs_btnode_prepare_change_key
- *  prepare to move contents of the block for old key to one of new key.
- *  the old buffer will not be removed, but might be reused for new buffer.
- *  it might return -ENOMEM because of memory allocation errors,
- *  and might return -EIO because of disk read errors.
+ * nilfs_btnode_prepare_change_key - prepare to change the search key of a
+ *                                   b-tree node block
+ * @btnc: page cache in which the b-tree node block is buffered
+ * @ctxt: structure for exchanging context information for key change
+ *
+ * nilfs_btnode_prepare_change_key() prepares to move the contents of the
+ * b-tree node block of the old key given in the "oldkey" member of @ctxt to
+ * the position of the new key given in the "newkey" member of @ctxt in the
+ * page cache @btnc.  Here, the key of the block is an index in units of
+ * blocks, and if the page and block sizes match, it matches the page index
+ * in the page cache.
+ *
+ * If the page size and block size match, this function attempts to move the
+ * entire folio, and in preparation for this, inserts the original folio into
+ * the new index of the cache.  If this insertion fails or if the page size
+ * and block size are different, it falls back to a copy preparation using
+ * nilfs_btnode_create_block(), inserts a new block at the position
+ * corresponding to "newkey", and stores the buffer head pointer in the
+ * "newbh" member of @ctxt.
+ *
+ * Note that the current implementation does not support folio sizes larger
+ * than the page size.
+ *
+ * Return: 0 on success, or the following negative error code on failure.
+ * * %-EIO	- I/O error (metadata corruption).
+ * * %-ENOMEM	- Insufficient memory available.
  */
 int nilfs_btnode_prepare_change_key(struct address_space *btnc,
 				    struct nilfs_btnode_chkey_ctxt *ctxt)
@@ -245,8 +266,21 @@ int nilfs_btnode_prepare_change_key(struct address_space *btnc,
 }
 
 /**
- * nilfs_btnode_commit_change_key
- *  commit the change_key operation prepared by prepare_change_key().
+ * nilfs_btnode_commit_change_key - commit the change of the search key of
+ *                                  a b-tree node block
+ * @btnc: page cache in which the b-tree node block is buffered
+ * @ctxt: structure for exchanging context information for key change
+ *
+ * nilfs_btnode_commit_change_key() executes the key change based on the
+ * context @ctxt prepared by nilfs_btnode_prepare_change_key().  If no valid
+ * block buffer is prepared in "newbh" of @ctxt (i.e., a full folio move),
+ * this function removes the folio from the old index and completes the move.
+ * Otherwise, it copies the block data and inherited flag states of "oldbh"
+ * to "newbh" and clears the "oldbh" from the cache.  In either case, the
+ * relocated buffer is marked as dirty.
+ *
+ * As with nilfs_btnode_prepare_change_key(), the current implementation does
+ * not support folio sizes larger than the page size.
  */
 void nilfs_btnode_commit_change_key(struct address_space *btnc,
 				    struct nilfs_btnode_chkey_ctxt *ctxt)
@@ -285,8 +319,19 @@ void nilfs_btnode_commit_change_key(struct address_space *btnc,
 }
 
 /**
- * nilfs_btnode_abort_change_key
- *  abort the change_key operation prepared by prepare_change_key().
+ * nilfs_btnode_abort_change_key - abort the change of the search key of a
+ *                                 b-tree node block
+ * @btnc: page cache in which the b-tree node block is buffered
+ * @ctxt: structure for exchanging context information for key change
+ *
+ * nilfs_btnode_abort_change_key() cancels the key change associated with the
+ * context @ctxt prepared via nilfs_btnode_prepare_change_key() and performs
+ * any necessary cleanup.  If no valid block buffer is prepared in "newbh" of
+ * @ctxt, this function removes the folio from the destination index and aborts
+ * the move.  Otherwise, it clears "newbh" from the cache.
+ *
+ * As with nilfs_btnode_prepare_change_key(), the current implementation does
+ * not support folio sizes larger than the page size.
  */
 void nilfs_btnode_abort_change_key(struct address_space *btnc,
 				   struct nilfs_btnode_chkey_ctxt *ctxt)