linux-kernel - Re: [PATCH] fs: prefer read_iter over read and write

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Message-ID: <YogIg9FG7C/f98bf@zx2c4.com>
Date:   Fri, 20 May 2022 23:30:43 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] fs: prefer read_iter over read and write_iter over write

Hi David,

On Fri, May 20, 2022 at 09:24:50PM +0000, David Laight wrote:
> From: Jason A. Donenfeld
> > Sent: 20 May 2022 14:51
> > 
> > Most kernel code prefers read_iter over read and write_iter over write,
> > yet the read function pointer is tested first. Reverse these so that the
> > iter function is always used first.
> 
> There will be a measurable performance hit for the xxx_iter versions.

Indeed. We now have the misfortune of a 3% hit on random.c, per this
sub-thread:

   https://lore.kernel.org/lkml/Yoey+FOYO69lS5qP@zx2c4.com/

The hope is that it eventually becomes faster... :-\

Jason