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

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Message-ID: <6f7ba1dbbc4749f09ae44aafc6ada284@AcuMS.aculab.com>
Date:   Wed, 27 Jan 2021 16:28:38 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Pavel Begunkov' <asml.silence@...il.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

From: Pavel Begunkov
> Sent: 27 January 2021 15:48
> 
> On 16/01/2021 05:18, Al Viro wrote:
> > On Sat, Jan 09, 2021 at 10:11:09PM +0000, Pavel Begunkov wrote:
> >
> >>> 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.
> >
> > Reasonable, and from what I remember from looking through the users,
> > no readers will bother with looking at both at the same time.
> 
> Al, are you going turn it into a patch, or prefer me to take over?

I'd definitely leave the type as a bitmap.

It may be useful to add ITER_IOVEC_SINGLE to optimise some
very common paths for user iovec with only a single buffer.
But you'd probably want to keep the full version for
more unusual (or already expensive) cases.

	David

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