linux-kernel - Re: [PATCH] iov_iter: optimise iter type checking

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Date:   Tue, 12 Jan 2021 16:04:51 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     David Laight <David.Laight@...LAB.COM>,
        'Al Viro' <viro@...iv.linux.org.uk>
Cc:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iov_iter: optimise iter type checking

On 11/01/2021 09:35, David Laight wrote:
> From: Pavel Begunkov
>> Sent: 09 January 2021 22:11
> ....
>>> Does any code actually look at the fields as a pair?
>>> Would it even be better to use separate bytes?
>>> Even growing the on-stack structure by a word won't really matter.
>>
>> u8 type, rw;
>>
>> That won't bloat the struct. I like the idea. If used together compilers
>> can treat it as u16.
>>
>> btw there is a 4B hole just after for x64.
> 
> I've just had a quick look at the sources.
> (Nothing was powered up earlier.)
> 
> AFAICT nothing needs the RW flag to be in the same word
> as the type.
> If you think about it, the call site is specific to read/write.
> The only places iov_iter_rw() is used in inside helper functions
> to save the direction being passed from the caller.
> 
> I hope the comment about bit 1 being BVEC_FLAG_NO_REF is old.
> I can't find any references to that flag.

Yep, long dead.

-- 
Pavel Begunkov