linux-kernel - Re: [PATCH] eventfs: Stop using dcache

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Message-ID: <CAHk-=wiWbqy-hd-Xo6dwx=6dH+gMyvf_wFtih7eEpBkHyigzJA@mail.gmail.com>
Date: Wed, 3 Jan 2024 14:14:08 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>, 
	Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Al Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, linux-fsdevel@...r.kernel.org, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] eventfs: Stop using dcache_readdir() for getdents()

On Wed, 3 Jan 2024 at 14:04, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> I actually have something almost working too. Here's the WIP. It only works
> for tracefs, and now eventfs needs to be updated as the "events" directory
> no longer has the right ownership. So I need a way to link the eventfs
> entries to use the tracefs default conditionally.

Ack. So the ->getattr() and ->permission() thing is a bit more
targeted to "look at modes", and is probably better just for that
reason.

Doing it in d_revalidate() is a bit hacky, and impacts path lookup a
bit even when not necessary. But it's still a lot less evil than
walking the dentry tree manually.

So that d_revalidate() patch was more of a "I think you can make it
smaller by just hooking in at this layer"). So d_revalidate ends up
with a smaller patch, I think, but it has the problem that now you
*have* to be able to deal with things in RCU context.

In contrast, doing it in ->getattr() and ->permission() ends up
meaning you can use sleeping locks etc if you need to serialize, for
example. So it's a bit more specific to the whole issue of "deal with
modes and owndership", but it is *also* a bit more flexible in how you
can then do it.

Anyway, your patch looks fine from a quick scan.

                  Linus