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Sun, 12 Oct 2025 05:58:23 -0700 (PDT) From: Askar Safin To: luca.boccassi@gmail.com Cc: alx@kernel.org, brauner@kernel.org, cyphar@cyphar.com, linux-fsdevel@vger.kernel.org, linux-man@vger.kernel.org, safinaskar@gmail.com Subject: Re: [PATCH] man/man2/move_mount.2: document EINVAL on multiple instances Date: Sun, 12 Oct 2025 15:58:02 +0300 Message-ID: <20251012125819.136942-1-safinaskar@gmail.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Okay, I spent some more time researching this. By default move_mount should work in your case. But if we try to move mount, residing under shared mount, then move_mount will not work. This is documented here: https://elixir.bootlin.com/linux/v6.17/source/Documentation/filesystems/sharedsubtree.rst#L497 "/" is shared by default if we booted using systemd. This is why you observing EINVAL. I just found that this is already documented in move_mount(2): EINVAL The source mount object's parent mount has shared mount propagation, and thus cannot be moved (as described in mount_name‐ spaces(7)). So everything is working as intended, and no changes to manual pages are needed. On the other hand, this is a good idea to add a bigger warning to move_mount(2) (and to mount(2), it is affected, too). I. e. to add something like this to main text of move_mount (as opposed to "ERRORS"): "Note that systemd makes "/" shared by default. Moving mounts residing under shared mounts is prohibited, so attempting to move attached mount using move_mount likely will not work". (I personally don't have plans to submit this as a patch.) -- Askar Safin