Re: [PATCH 1/1] man2/mount.2: expand and clarify docs for MS_REMOUNT | MS_BIND
From: Aleksa Sarai <hidden>
Date: 2025-08-22 13:06:25
Also in:
linux-fsdevel, linux-man
On 2025-08-22, Askar Safin [off-list ref] wrote:
quoted hunk ↗ jump to hunk
My edit is based on experiments and reading Linux code Signed-off-by: Askar Safin <redacted> --- man/man2/mount.2 | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-)diff --git a/man/man2/mount.2 b/man/man2/mount.2 index 5d83231f9..909b82e88 100644 --- a/man/man2/mount.2 +++ b/man/man2/mount.2@@ -405,7 +405,19 @@ flag can be used with to modify only the per-mount-point flags. .\" See https://lwn.net/Articles/281157/ This is particularly useful for setting or clearing the "read-only" -flag on a mount without changing the underlying filesystem. +flag on a mount without changing flags of the underlying filesystem.
For obvious reasons, I would prefer the term "filesystem parameters" here but mount(2) is kind of loose with its terminology...
+The +.I data +argument is ignored if +.B MS_REMOUNT +and +.B MS_BIND +are specified. +The +.I mountflags +should specify existing per-mount-point flags, +except for those parameters +that are deliberately changed.
I would phrase this more like a note to make the advice a bit clearer: Note that the mountpoint will have its existing per-mount-point flags cleared and replaced with those in .I mountflags when .B MS_REMOUNT and .B MS_BIND are specified. This means that if you wish to preserve any existing per-mount-point flags (which can be retrieved using .BR statfs (2)), you need to include them in .IR mountflags , along with the per-mount-point flags you wish to set (or with the flags you wish to clear missing). (Still a bit too wordy, there's probably a nicer way of writing it...) It might also be a good idea to reference locked mount flags (which are explained in more detail in mount_namespaces(7)) since they are very relevant to the text you're adding about MS_REMOUNT|MS_BIND. The current docs only mention locked mounts once and the description is kind of insufficient (it implies that only MS_REMOUNT affects this, and that it's related to the mount being locked -- neither is really true). When dealing with a mount with locked flags, remembering to include existing mount attributes is very important.
quoted hunk ↗ jump to hunk
Specifying .I mountflags as:@@ -416,8 +428,11 @@ MS_REMOUNT | MS_BIND | MS_RDONLY .EE .in .P -will make access through this mountpoint read-only, without affecting -other mounts. +will make access through this mountpoint read-only +(and clear all other per-mount-point flags),
(clearing all other per-mount-point flags)
+without affecting +other mounts +of this filesystem. .\" .SS Creating a bind mount If
-- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH https://www.cyphar.com/
Attachments
- signature.asc [application/pgp-signature] 265 bytes