linux-kernel - Re: [PATCH] eventfs: Have inodes have unique inode numbers

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Date: Sun, 28 Jan 2024 17:42:30 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	LKML <linux-kernel@...r.kernel.org>, 
	Linux Trace Devel <linux-trace-devel@...r.kernel.org>, Christian Brauner <brauner@...nel.org>, 
	Ajay Kaher <ajay.kaher@...adcom.com>, Geert Uytterhoeven <geert@...ux-m68k.org>, 
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] eventfs: Have inodes have unique inode numbers

On Sun, 28 Jan 2024 at 17:00, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>    mkdir dummy
>    cd dummy
>    echo "Hello" > hello
>    ( sleep 10; cat ) < hello &
>    rm hello
>    cd ..
>    rmdir dummy

Note that it's worth repeating that simple_recursive_removal()
wouldn't change any of the above. It only unhashes things and makes
them *look* gone, doing things like clearing i_nlink etc.

But those VFS data structures would still exist, and the files that
had them open would still continue to be open.

So if you thought that simple_recursive_removal() would make the above
kind of thing not able to happen, and that eventfs wouldn't have to
deal with dentries that point to event_inodes that are dead, you were
always wrong.

simple_recursive_removal() is mostly just lipstick on a pig. It does
cause the cached dentries that have no active use be removed earlier,
so it has that "memory pressure" kind of effect, but it has no real
fundamental semantic effect.

Of course, for a filesystem where the dentry tree *is* the underlying
data (ie the 'tmpfs' kind, but also things like debugfs or ipathfs,
for example), then things are different.

There the dentries are the primary thing, and not just a cache in
front of the backing store.

But you didn't want that, and those days are long gone as far as
tracefs is concerned.

              Linus