linux-kernel - Re: [for-linus][PATCH 1/3] eventfs: Have the inodes all for files and directories all be the same

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Date: Mon, 22 Jan 2024 11:38:52 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Christian Brauner <brauner@...nel.org>, 
	Al Viro <viro@...iv.linux.org.uk>, Ajay Kaher <ajay.kaher@...adcom.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [for-linus][PATCH 1/3] eventfs: Have the inodes all for files and
 directories all be the same

Hi Stephen,

On Wed, Jan 17, 2024 at 3:37 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
>
> The dentries and inodes are created in the readdir for the sole purpose of
> getting a consistent inode number. Linus stated that is unnecessary, and
> that all inodes can have the same inode number. For a virtual file system
> they are pretty meaningless.
>
> Instead use a single unique inode number for all files and one for all
> directories.
>
> Link: https://lore.kernel.org/all/20240116133753.2808d45e@gandalf.local.home/
> Link: https://lore.kernel.org/linux-trace-kernel/20240116211353.412180363@goodmis.org
>
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: Al  Viro <viro@...IV.linux.org.uk>
> Cc: Ajay Kaher <ajay.kaher@...adcom.com>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Thanks for your patch, which is now commit 53c41052ba312176 ("eventfs:
Have the inodes all for files and directories all be the same") in
v6.8-rc1, to which I have bisected the issue below.

> --- a/fs/tracefs/event_inode.c
> +++ b/fs/tracefs/event_inode.c
> @@ -32,6 +32,10 @@
>   */
>  static DEFINE_MUTEX(eventfs_mutex);
>
> +/* Choose something "unique" ;-) */
> +#define EVENTFS_FILE_INODE_INO         0x12c4e37
> +#define EVENTFS_DIR_INODE_INO          0x134b2f5
> +
>  /*
>   * The eventfs_inode (ei) itself is protected by SRCU. It is released from
>   * its parent's list and will have is_freed set (under eventfs_mutex).
> @@ -352,6 +356,9 @@ static struct dentry *create_file(const char *name, umode_t mode,
>         inode->i_fop = fop;
>         inode->i_private = data;
>
> +       /* All files will have the same inode number */
> +       inode->i_ino = EVENTFS_FILE_INODE_INO;
> +
>         ti = get_tracefs(inode);
>         ti->flags |= TRACEFS_EVENT_INODE;
>         d_instantiate(dentry, inode);
> @@ -388,6 +395,9 @@ static struct dentry *create_dir(struct eventfs_inode *ei, struct dentry *parent
>         inode->i_op = &eventfs_root_dir_inode_operations;
>         inode->i_fop = &eventfs_file_operations;
>
> +       /* All directories will have the same inode number */
> +       inode->i_ino = EVENTFS_DIR_INODE_INO;
> +
>         ti = get_tracefs(inode);
>         ti->flags |= TRACEFS_EVENT_INODE;

This confuses "find".
Running "find /sys/" now prints lots of error messages to stderr:

find: File system loop detected;
‘/sys/kernel/debug/tracing/events/initcall/initcall_finish’ is part of
the same file system loop as
‘/sys/kernel/debug/tracing/events/initcall’.
find: File system loop detected;
‘/sys/kernel/debug/tracing/events/initcall/initcall_start’ is part of
the same file system loop as
‘/sys/kernel/debug/tracing/events/initcall’.
find: File system loop detected;
‘/sys/kernel/debug/tracing/events/initcall/initcall_level’ is part of
the same file system loop as
‘/sys/kernel/debug/tracing/events/initcall’.
[...]

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68korg

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds