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

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Date: Sat, 27 Jan 2024 13:47:32 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: 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 Fri, 26 Jan 2024 at 13:49, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> For example, what purpose does 'e->dentry' and 'ei->d_childen[]' have?
> Isn't that entirely a left-over from the bad old days?
>
> So please try to look at things to *fix* and simplify, not at things
> to mess around with and make more complicated.

So here's an attempt at some fairly trivial but entirely untested cleanup.

I have *not* tested this at all, and I assume you have some extensive
test-suite that you run. So these are "signed-off' in the sense that
the patch looks fine, it builds in one configuration for me, but maybe
there's something really odd going on.

The first patch is trivial dead code removal.

The second patch is because I really do not understand the reason for
the 'ei->dentry' pointer, and it just looks messy.

It looks _particularly_ messy when it is mixed up in operations that
really do not need it and really shouldn't use it.

The eventfs_find_events() code was using the dentry parent pointer to
find the parent (fine, and simple), then looking up the tracefs inode
using that (fine so far), but then it looked up the dentry using
*that*. But it already *had* the dentry - it's that parent dentry it
just used to find the tracefs inode. The code looked nonsensical.

Similarly, it then (in the set_top_events_ownership() helper) used
'ei->dentry' to update the events attr, but all that really wants is
the superblock root. So instead of passing a dentry, just pass the
superblock pointer, which you can find in either the dentry or in the
VFS inode, depending on which level you're working at.

There are tons of other 'ei->dentry' uses, and I didn't look at those.
Baby steps. But this *seems* like an obvious cleanup, and many small
obvious cleanups later and perhaps the 'ei->dentry' pointer (and the
'->d_children[]' array) can eventually go away. They should all be
entirely useless - there's really no reason for a filesystem to hold
on to back-pointers of dentries.

Anybody willing to run the test-suite on this?

                    Linus

View attachment "0001-tracefs-remove-stale-update_gid-code.patch" of type "text/x-patch" (2612 bytes)

View attachment "0002-tracefs-avoid-using-the-ei-dentry-pointer-unnecessar.patch" of type "text/x-patch" (3561 bytes)