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docs: path-lookup: update i_op->put_link and cookie description

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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: commit fceef393a5 ("switch ->get_link() to
delayed_call, kill ->put_link()")

Signed-off-by: Fox Chen <foxhlchen@gmail.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20210527091618.287093-9-foxhlchen@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Fox Chen 2021-05-27 17:16:13 +08:00 committed by Jonathan Corbet
parent 4a00e4bd59
commit 671f73356f
1 changed files with 8 additions and 22 deletions
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30
Documentation/filesystems/path-lookup.rst
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@ -1066,34 +1066,20 @@ 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. 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()``. Later 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