From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751205AbeAVT6d (ORCPT <rfc822;w@1wt.eu>);
        Mon, 22 Jan 2018 14:58:33 -0500
Received: from mail-io0-f194.google.com ([209.85.223.194]:39048 "EHLO
        mail-io0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750891AbeAVT6L (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Mon, 22 Jan 2018 14:58:11 -0500
X-Google-Smtp-Source: AH8x224xBE+xA/HvfXmIjIKKIpoocdy9hIsg/VArQHOyEtVgit6QsM33aPjSvRcuV7w7KNB6Q7DW6J2mFrA2dqjS+A0=
MIME-Version: 1.0
In-Reply-To: <6f0abb8d-84b3-070e-fca7-c71524311702@prevas.dk>
References: <201801170233.JDG21842.OFOJMQSHtOFFLV@I-love.SAKURA.ne.jp>
 <CA+55aFyxyjN0Mqnz66B4a0R+uR8DdfxdMhcg5rJVi8LwnpSRfA@mail.gmail.com>
 <201801172008.CHH39543.FFtMHOOVSQJLFO@I-love.SAKURA.ne.jp>
 <201801181712.BFD13039.LtHOSVMFJQFOFO@I-love.SAKURA.ne.jp>
 <20180118122550.2lhsjx7hg5drcjo4@node.shutemov.name> <d8347087-18a6-1709-8aa8-3c6f2d16aa94@linux.intel.com>
 <20180118145830.GA6406@redhat.com> <20180118165629.kpdkezarsf4qymnw@node.shutemov.name>
 <CA+55aFy43ypm0QvA5SqNR4O0ZJETbkR3NDR=dnSdvejc_nmSJQ@mail.gmail.com>
 <20180118234955.nlo55rw2qsfnavfm@node.shutemov.name> <20180119125503.GA2897@bombadil.infradead.org>
 <CA+55aFwWCeFrhN+WJDD8u9nqBzmvknXk428Q0dVwwXAvwhg_-w@mail.gmail.com> <6f0abb8d-84b3-070e-fca7-c71524311702@prevas.dk>
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 22 Jan 2018 11:58:09 -0800
X-Google-Sender-Auth: kyS4RDvKtJXD1yXBPN8v5_zb1Xg
Message-ID: <CA+55aFyytOcFQ0Y3GOzGo7ozchM1kGfJgoAHgHpETLP9qKwPAw@mail.gmail.com>
Subject: Re: [mm 4.15-rc8] Random oopses under memory pressure.
To: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Jan 22, 2018 at 5:26 AM, Rasmus Villemoes
<rasmus.villemoes@prevas.dk> wrote:
> 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?

Hmm. I have definitely seen gcc generate the difference with a divide.

But it may be less of a "can't do it with a multiply" and more of a
"not all versions of gcc have the multiplication optimization".

            Linus