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The comment on top of __lookup_mnt() states that it finds the first mount implying that there could be multiple mounts mounted at the same dentry with the same parent. On older kernels "shadow mounts" could be created during mount propagation. So if a mount @m in the destination propagation tree already had a child mount @p mounted at @mp then any mount @n we propagated to @m at the same @mp would be appended after the preexisting mount @p in @mount_hashtable. This was a completely direct way of creating shadow mounts. That direct way is gone but there are still subtle ways to create shadow mounts. For example, when attaching a source mnt @mnt to a shared mount. The root of the source mnt @mnt might be overmounted by a mount @o after we finished path lookup but before we acquired the namespace semaphore to copy the source mount tree @mnt. After we acquired the namespace lock @mnt is copied including @o covering it. After we attach @mnt to a shared mount @dest_mnt we end up propagation it to all it's peer and slaves @d. If @d already has a mount @n mounted on top of it we tuck @mnt beneath @n. This means, we mount @mnt at @d and mount @n on @mnt. Now we have both @o and @n mounted on the same mountpoint at @mnt. Explain this in the documentation as this is pretty subtle. Reviewed-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org> Message-Id: <20230202-fs-move-mount-replace-v4-2-98f3d80d7eaa@kernel.org> Signed-off-by: Christian Brauner <brauner@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.