Re: [PATCH v2 08/12] docs: path-lookup: update i_op->put_link and cookie description

From: NeilBrown
Date: Sun Apr 18 2021 - 21:37:59 EST

On Tue, Mar 16 2021, Fox Chen wrote:

> 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 fceef393a538 ("switch ->get_link() to
> delayed_call, kill ->put_link()")
> Signed-off-by: Fox Chen <foxhlchen@xxxxxxxxx>
> ---
> Documentation/filesystems/path-lookup.rst | 30 ++++++-----------------
> 1 file changed, 8 insertions(+), 22 deletions(-)
> diff --git a/Documentation/filesystems/path-lookup.rst b/Documentation/filesystems/path-lookup.rst
> index e6b6c43ff0f6..8ab95dd9046e 100644
> --- a/Documentation/filesystems/path-lookup.rst
> +++ b/Documentation/filesystems/path-lookup.rst
> @@ -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,

I'd put a ":", not "," at the end of above line.

> +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

() after function names please, both above and below.

Also: "when VFS want to put the link"

With these changes:
Reviewed-by: NeilBrown <neilb@xxxxxxx>


> +``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
> --
> 2.30.2

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