[PATCH 08/12] docs: path-lookup: update i_op->put_link and cookie description

Fox Chen
2021-01-26T17:36:46Z

No inode->put_link operation anymore. We use delayed_call to
deal with link destruction. Cookie has been replaced with
struct delayed_call.

Related commit: fceef393a538134f03b778c5d2519e670269342f

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
---
Documentation/filesystems/path-lookup.rst | 31 +++++++----------------
1 file changed, 9 insertions(+), 22 deletions(-)

diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
index 0a362849b26f..8572300b5405 100644
--- a/Documentation/filesystems/path-lookup.rst
+++ b/Documentation/filesystems/path-lookup.rst
@@ -1068,34 +1068,21 @@ method. This is called both in RCU-walk and REF-walk. In RCU-walk the
RCU-walk. Much like the ``i_op->permission()`` method we
looked at previously, ``->get_link()`` would need to be careful that
all the data structures it references are safe to be accessed while
-holding no counted reference, only the RCU lock. Though getting a
-reference with ``->follow_link()`` is not yet done in RCU-walk mode, the
-code is ready to release the reference when that does happen.
-
-This need to drop the reference to a symlink adds significant
-complexity. It requires a reference to the inode so that the
-``i_op->put_link()`` inode operation can be called. In REF-walk, that
-reference is kept implicitly through a reference to the dentry, so
-keeping the ``struct path`` of the symlink is easiest. For RCU-walk,
-the pointer to the inode is kept separately. To allow switching from
-RCU-walk back to REF-walk in the middle of processing nested symlinks
-we also need the seq number for the dentry so we can confirm that
-switching back was safe.
-
-Finally, when providing a reference to a symlink, the filesystem also
-provides an opaque "cookie" that must be passed to ``->put_link()`` so that it
-knows what to free. This might be the allocated memory area, or a
-pointer to the ``struct page`` in the page cache, or something else
-completely. Only the filesystem knows what it is.
+holding no counted reference, only the RCU lock.
+
+Finally, a callback ``struct delayed_called`` will be passed to get_link,
+file systems can set their own put_link function and argument through
+``set_delayed_call``. Latter on, when vfs wants to put link, it will call
+``do_delayed_call`` to invoke that callback function with the argument.

In order for the reference to each symlink to be dropped when the walk completes,
whether in RCU-walk or REF-walk, the symlink stack needs to contain,
along with the path remnants:

-- the ``struct path`` to provide a reference to the inode in REF-walk
-- the ``struct inode *`` to provide a reference to the inode in RCU-walk
+- the ``struct path`` to provide a reference to the previous path
+- the ``const char *`` to provide a reference to the to previous name
- the ``seq`` to allow the path to be safely switched from RCU-walk to REF-walk
-- the ``cookie`` that tells ``->put_path()`` what to put.
+- the ``struct delayed_call`` for later invocation.

This means that each entry in the symlink stack needs to hold five
pointers and an integer instead of just one pointer (the path
--
2.30.0