linux-kernel - Re: [RFC PATCH 3/3] treewide: pipe: Convert all references to pipe->{head,tail,max_usage,ring

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAHk-=wg10syD6-3BwuQCCKxua3_bdK1gfjiw_DtCqNqe8zXFaA@mail.gmail.com>
Date: Thu, 6 Mar 2025 08:04:18 -1000
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: Oleg Nesterov <oleg@...hat.com>, Rasmus Villemoes <ravi@...vas.dk>, Miklos Szeredi <miklos@...redi.hu>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Jan Kara <jack@...e.cz>, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, Mateusz Guzik <mjguzik@...il.com>, 
	"Gautham R. Shenoy" <gautham.shenoy@....com>, Neeraj.Upadhyay@....com, Ananth.narayan@....com, 
	Swapnil Sapkal <swapnil.sapkal@....com>
Subject: Re: [RFC PATCH 3/3] treewide: pipe: Convert all references to
 pipe->{head,tail,max_usage,ring_size} to unsigned short

On Thu, 6 Mar 2025 at 05:33, K Prateek Nayak <kprateek.nayak@....com> wrote:
> >>
> >> I dunno... but if we do this, perhaps we should
> >> s/unsigned int/pipe_index_t instead?
> >>
> >> At least this would be more grep friendly.
>
> Ack. I'll leave the typedef untouched and convert these to use
> pipe_index_t. This was an experiment so see if anything breaks with u16
> conversion just to get more testing on that scenario. As Rasmus
> mentioned, leaving the head and tail as u32 on 64bit will lead to
> better code generation.

Yes, I was going to say the same - please don't change to 'unsigned short'.

Judicious use of 'pipe_index_t' may be a good idea, but as I fixed
some issues Rasmus found, I was also looking at the generated code,
and on at least x86 where 16-bit generates extra instructions and
prefixes, it seems marginally better to treat the values as 32-bit,
and then only do the compares in 16 bits.

That only causes a few "movzwl" instructions (at load time), and then
the occasional "cmpw" (empty check) and "movw" (store) etc.

But I only did a very quick "let's look at a few cases of x86-64 also
using a 16-bit pipe_index_t".

So for testing purposes your patch looks fine, but not as something to apply.

If anything, I think we should actively try to remove as many direct
accesses to these pipe fields as humanly possible. As Oleg said, a lot
of them should just be cleaned up to use the helpers we already have.

Rasmus found a few cases of that already, like that FIONREAD case
where it was just doing a lot of open-coding of things that shouldn't
be open-coded.

I've fixed the two cases he pointed at up as obvious bugs, but it
would be good to see where else issues like this might lurk.

                 Linus