linux-kernel - RE: [RFC,iov_iter v2 3/8] iov_iter: add copyin

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Message-ID: <8f6771a01fe74cde86641e780d31a2ce@AcuMS.aculab.com>
Date:   Mon, 13 Jun 2022 07:53:19 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Joe Damato' <jdamato@...tly.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC,iov_iter v2 3/8] iov_iter: add copyin_iovec helper

From: Joe Damato
> Sent: 12 June 2022 09:58
> 
> copyin_iovec is a helper which wraps copyin and selects the right copy
> method based on the iter_copy_type.

A pretty bad description.

> Signed-off-by: Joe Damato <jdamato@...tly.com>
> ---
>  lib/iov_iter.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index d32d7e5..6720cb2 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -168,6 +168,15 @@ static int copyin(void *to, const void __user *from, size_t n)
>  	return n;
>  }
> 
> +static int copyin_iovec(void *to, const void __user *from, size_t n,
> +			struct iov_iter *i)
> +{
> +	if (unlikely(iov_iter_copy_is_nt(i)))
> +		return __copy_from_user_nocache(to, from, n);
> +	else
> +		return copyin(to, from, n);
> +}

Isn't this extra conditional going to have a measurable impact
on all the normal copy paths?

The additional costs of all the 'iovec' types is bad enough
already.

	David

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