linux-kernel - Re: [PATCH v3 3/3] mm: don't miss the last page because of round-off error

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Date:   Wed, 5 Sep 2018 14:08:18 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Roman Gushchin <guro@...com>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <kernel-team@...com>, Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, Rik van Riel <riel@...riel.com>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 3/3] mm: don't miss the last page because of
 round-off error

On Wed, 29 Aug 2018 14:33:19 -0700 Roman Gushchin <guro@...com> wrote:

> >From d8237d3df222e6c5a98a74baa04bc52edf8a3677 Mon Sep 17 00:00:00 2001
> From: Roman Gushchin <guro@...com>
> Date: Wed, 29 Aug 2018 14:14:48 -0700
> Subject: [PATCH] math64: prevent double calculation of DIV64_U64_ROUND_UP()
>  arguments
> 
> Cause the DIV64_U64_ROUND_UP(ll, d) macro to cache
> the result of (d) expression in a local variable to
> avoid double calculation, which might bring unexpected
> side effects.
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> ---
>  include/linux/math64.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/math64.h b/include/linux/math64.h
> index 94af3d9c73e7..bb2c84afb80c 100644
> --- a/include/linux/math64.h
> +++ b/include/linux/math64.h
> @@ -281,6 +281,7 @@ static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor)
>  }
>  #endif /* mul_u64_u32_div */
>  
> -#define DIV64_U64_ROUND_UP(ll, d)	div64_u64((ll) + (d) - 1, (d))
> +#define DIV64_U64_ROUND_UP(ll, d)	\
> +	({ u64 _tmp = (d); div64_u64((ll) + _tmp - 1, _tmp); })
>  
>  #endif /* _LINUX_MATH64_H */

Does it have to be done as a macro?  A lot of these things are
implemented as nice inline C functions.

Also, most of these functions and macros return a value whereas
DIV64_U64_ROUND_UP() does not.  Desirable?

(And we're quite pathetic about documenting what those return values
_are_, which gets frustrating for the poor schmucks who sit here
reviewing code all day).