[PATCH v2] fanotify.7,fanotify_mark.2: update documentation of fanotify w.r.t fsid

[Amir Goldstein <amir73il@gmail.com>]

Clarify the conditions for getting the -EXDEV and -ENODEV errors.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Changes since v1:
- Fix review comments [1]

[1] https://lore.kernel.org/linux-fsdevel/20241101130732.xzpottv5ru63w4wd@devuan/

 man/man2/fanotify_mark.2 | 27 +++++++++++++++++++++------
 man/man7/fanotify.7      | 11 +++++++++++
 2 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/man/man2/fanotify_mark.2 b/man/man2/fanotify_mark.2
index fc9b83459..47cafb21c 100644
--- a/man/man2/fanotify_mark.2
+++ b/man/man2/fanotify_mark.2
@@ -659,17 +659,16 @@ The filesystem object indicated by
 .I dirfd
 and
 .I pathname
-is not associated with a filesystem that supports
+is associated with a filesystem that reports zero
 .I fsid
 (e.g.,
 .BR fuse (4)).
-.BR tmpfs (5)
-did not support
-.I fsid
-prior to Linux 5.13.
-.\" commit 59cda49ecf6c9a32fae4942420701b6e087204f6
 This error can be returned only with an fanotify group that identifies
 filesystem objects by file handles.
+Since Linux 6.8,
+.\" commit 30ad1938326bf9303ca38090339d948975a626f5
+this error can be returned
+when trying to add a mount or filesystem mark.
 .TP
 .B ENOENT
 The filesystem object indicated by
@@ -768,6 +767,22 @@ which uses a different
 than its root superblock.
 This error can be returned only with an fanotify group that identifies
 filesystem objects by file handles.
+Since Linux 6.8,
+.\" commit 30ad1938326bf9303ca38090339d948975a626f5
+this error will be returned
+when trying to add a mount or filesystem mark on a subvolume,
+when trying to add inode marks in different subvolumes,
+or when trying to add inode marks in a
+.BR btrfs (5)
+subvolume and in another filesystem.
+Since Linux 6.8,
+.\" commit 30ad1938326bf9303ca38090339d948975a626f5
+this error will also be returned
+when trying to add marks in different filesystems,
+where one of the filesystems reports zero
+.I fsid
+(e.g.,
+.BR fuse (4)).
 .SH STANDARDS
 Linux.
 .SH HISTORY
diff --git a/man/man7/fanotify.7 b/man/man7/fanotify.7
index 449af949c..b270f3c99 100644
--- a/man/man7/fanotify.7
+++ b/man/man7/fanotify.7
@@ -575,6 +575,17 @@ and contains the same value as
 .I f_fsid
 when calling
 .BR statfs (2).
+Note that some filesystems (e.g.,
+.BR fuse (4))
+report zero
+.IR fsid .
+In these cases,
+it is not possible to use
+.I fsid
+to associate the event with a specific filesystem instance,
+so monitoring different filesystem instances that report zero
+.I fsid
+with the same fanotify group is not supported.
 .TP
 .I handle
 This field contains a variable-length structure of type
-- 
2.34.1