linux-kernel - Re: [PATCH 16/16] mm: pass get_user_pages_fast iterator arguments in a structure

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Message-ID: <CAHk-=wjSo+TzkvYnAqrp=eFgzzc058DhSMTPr4-2quZTbGLfnw@mail.gmail.com>
Date:   Tue, 11 Jun 2019 15:09:24 -1000
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Christoph Hellwig <hch@....de>,
        James Hogan <jhogan@...nel.org>,
        Paul Burton <paul.burton@...s.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-mips@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Linux-sh list <linux-sh@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>, sparclinux@...r.kernel.org,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH 16/16] mm: pass get_user_pages_fast iterator arguments in
 a structure

On Tue, Jun 11, 2019 at 2:55 PM Nicholas Piggin <npiggin@...il.com> wrote:
>
> What does this do for performance? I've found this pattern can be
> bad for store aliasing detection.

I wouldn't expect it to be noticeable, and the lack of argument
reloading etc should make up for it. Plus inlining makes it a
non-issue when that happens.

But I guess we could also at least look at using "restrict", if that
ends up helping. Unlike the completely bogus type-based aliasing rules
(that we disable because I think the C people were on some bad bad
drugs when they came up with them), restricted pointers are a real
thing that makes sense.

That said, we haven't traditionally used it, and I don't know how much
it helps gcc. Maybe gcc ignores it entirely? S

               Linus