netdev - Re: [PATCH] net: Shrink sock.sk_err sk_err

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Date:   Tue, 5 Jul 2022 16:01:23 +0300
From:   Leonard Crestez <cdleonard@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Soheil Hassas Yeganeh <soheil@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        Joanne Koong <joannelkoong@...il.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: Shrink sock.sk_err sk_err_soft to u16 from int

On 7/5/22 13:31, Paolo Abeni wrote:
> On Sun, 2022-07-03 at 23:06 +0300, Leonard Crestez wrote:
>> These fields hold positive errno values which are limited by
>> ERRNO_MAX=4095 so 16 bits is more than enough.
>>
>> They are also always positive; setting them to a negative errno value
>> can result in falsely reporting a successful read/write of incorrect
>> size.
>>
>> Signed-off-by: Leonard Crestez <cdleonard@...il.com>
>> ---
>>   include/net/sock.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> I ran some relatively complex tests without noticing issues but some corner
>> case where this breaks might exist.
> 
> Could you please explain in length the rationale behind this change?
> 
> Note that this additionally changes the struct sock binary layout,
> which in turn in quite relevant for high speed data transfer.

The rationale is that shrinking structs is almost always better. I know 
that due to various roundings it likely won't actually impact memory 
consumption unless accumulated with other size reductions.

These sk_err fields don't seem to be in a particularly "hot" area so I 
don't think it will impact performance.

My expectation is that after a socket error is reported the socket will 
likely be closed so that there will be very few writes to this field.

--
Regards,
Leonard