netdev - Re: [Patch bpf-next 3/4] skmsg: use bitfields for struct sk

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Message-ID: <87eczju30u.fsf@cloudflare.com>
Date: Thu, 27 Feb 2025 10:53:53 +0100
From: Jakub Sitnicki <jakub@...udflare.com>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: netdev@...r.kernel.org,  bpf@...r.kernel.org,  john.fastabend@...il.com,
  zhoufeng.zf@...edance.com,  zijianzhang@...edance.com,  Cong Wang
 <cong.wang@...edance.com>
Subject: Re: [Patch bpf-next 3/4] skmsg: use bitfields for struct sk_psock

On Wed, Feb 26, 2025 at 02:21 PM -08, Cong Wang wrote:
> On Wed, Feb 26, 2025 at 02:49:17PM +0100, Jakub Sitnicki wrote:
>> On Sat, Feb 22, 2025 at 10:30 AM -08, Cong Wang wrote:
>> > From: Cong Wang <cong.wang@...edance.com>
>> >
>> > psock->eval can only have 4 possible values, make it 8-bit is
>> > sufficient.
>> >
>> > psock->redir_ingress is just a boolean, using 1 bit is enough.
>> >
>> > Signed-off-by: Cong Wang <cong.wang@...edance.com>
>> > ---
>> >  include/linux/skmsg.h | 4 ++--
>> >  1 file changed, 2 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
>> > index bf28ce9b5fdb..beaf79b2b68b 100644
>> > --- a/include/linux/skmsg.h
>> > +++ b/include/linux/skmsg.h
>> > @@ -85,8 +85,8 @@ struct sk_psock {
>> >  	struct sock			*sk_redir;
>> >  	u32				apply_bytes;
>> >  	u32				cork_bytes;
>> > -	u32				eval;
>> > -	bool				redir_ingress; /* undefined if sk_redir is null */
>> > +	unsigned int			eval : 8;
>> > +	unsigned int			redir_ingress : 1; /* undefined if sk_redir is null */
>> >  	struct sk_msg			*cork;
>> >  	struct sk_psock_progs		progs;
>> >  #if IS_ENABLED(CONFIG_BPF_STREAM_PARSER)
>> 
>> Are you doing this bit packing to create a hole big enough to fit
>> another u32 introduced in the next patch?
>
> Kinda, or at least trying to save some space for the next patch. I am
> not yet trying to reorder them to make it more packed, because it can
> be a separate patch.

OK. Asking because the intention is not expressed in the description.

Nit: Why the switch to an implicitly sized integer type?
It feels a bit silly when you can just declare an `u8 eval`.