linux-kernel - Re: [mm 4.15-rc8] Random oopses under memory pressure.

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Date:   Mon, 22 Jan 2018 14:26:02 +0100
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...IV.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [mm 4.15-rc8] Random oopses under memory pressure.

On 2018-01-19 19:42, Linus Torvalds wrote:
> 
> I actually asked (long long ago) for an optinal compiler warning for
> "pointer subtraction with non-power-of-2 sizes". Not because of it
> being undefined, but simply because it's expensive. The
> divide->multiply thing doesn't always work,

Huh? If (compile-time constant, positive) d=m*2^k with m odd, and x is
known to be a multiple of d, x/d can always be computed as (x>>k)*m' ==
(x*m')>>k, with m' being the mod 2^N multiplicative inverse of m, right?
This works whether x has signed or unsigned type and whether it indeed
happens to be negative, AFAICT.

Sure, the multiplication by m' may not exactly be cheap, but one only
needs the low N bits of the result.

Rasmus