From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christian Brauner <brauner@kernel.org>
Subject: Re: [PATCH 87/87] fs: move i_blocks up a few places in struct inode
Date: Fri, 29 Sep 2023 11:32:49 +0200
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jeff Layton <jlayton@kernel.org>, Alexander Viro <viro@zeniv.linux.org.uk>, David Sterba <dsterba@suse.cz>, Amir Goldstein <amir73il@gmail.com>, Theodore Ts'o <tytso@mit.edu>, Eric Biederman <ebiederm@xmission.com>, Kees Cook <keescook@chromium.org>, Jeremy Kerr <jk@ozlabs.org>, Arnd Bergmann <arnd@arndb.de>, Michael Ellerman <mpe@ellerman.id.au>, Nicholas Piggin <npiggin@gmail.com>, Christophe Leroy <christophe.leroy@csgroup.eu>, Heiko Carstens <hca@linux.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>, Alexander Gordeev <agordeev@linux.ibm.com>, Christian Borntraeger <borntraeger@linux.ibm.com>, Sven Schnelle <svens@linux.ibm.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Arve =?utf-8?B?SGrDuG5uZXbDpWc=?= <arve@android.com>, Todd Kjos <>

On Thu, Sep 28, 2023 at 10:41:34AM -0700, Linus Torvalds wrote:
> On Thu, 28 Sept 2023 at 04:06, Jeff Layton <jlayton@kernel.org> wrote:
> >
> > Move i_blocks up above the i_lock, which moves the new 4 byte hole to
> > just after the timestamps, without changing the size of the structure.
> 
> I'm sure others have mentioned this, but 'struct inode' is marked with
> __randomize_layout, so the actual layout may end up being very
> different.
> 
> I'm personally not convinced the whole structure randomization is
> worth it - it's easy enough to figure out for any distro kernel since
> the seed has to be the same across machines for modules to work, so
> even if the seed isn't "public", any layout is bound to be fairly
> easily discoverable.
> 
> So the whole randomization only really works for private kernel
> builds, and it adds this kind of pain where "optimizing" the structure
> layout is kind of pointless depending on various options.
> 
> I certainly *hope* no distro enables that pointless thing, but it's a worry.

They don't last we checked. Just last cycle we moved stuff in struct
file around to optimize things and we explicitly said we don't give a
damn about struct randomization. Anyone who enables this will bleed
performance pretty badly, I would reckon.