linux-kernel - Re: [RFC][CFT][PATCHSET v1] uaccess unification

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Message-ID: <20170330184824.GS29622@ZenIV.linux.org.uk>
Date:   Thu, 30 Mar 2017 19:48:24 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Richard Henderson <rth@...ddle.net>,
        Will Deacon <will.deacon@....com>,
        Haavard Skinnemoen <hskinnemoen@...il.com>,
        Vineet Gupta <vgupta@...opsys.com>,
        Steven Miao <realmz6@...il.com>,
        Jesper Nilsson <jesper.nilsson@...s.com>,
        Mark Salter <msalter@...hat.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Richard Kuo <rkuo@...eaurora.org>,
        Tony Luck <tony.luck@...el.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        James Hogan <james.hogan@...tec.com>,
        Michal Simek <monstr@...str.eu>,
        David Howells <dhowells@...hat.com>,
        Ley Foon Tan <lftan@...era.com>,
        Jonas Bonn <jonas@...thpole.se>, Helge Deller <deller@....de>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Chen Liqin <liqin.linux@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Richard Weinberger <richard@....at>,
        Guan Xuetao <gxt@...c.pku.edu.cn>,
        Thomas Gleixner <tglx@...utronix.de>,
        Chris Zankel <chris@...kel.net>
Subject: Re: [RFC][CFT][PATCHSET v1] uaccess unification

On Thu, Mar 30, 2017 at 10:18:23AM -0700, Linus Torvalds wrote:

> This is all going in the wrong direction entirely.

This is not going into the tree - it's just a "let's check your
theory about might_fault() overhead being the source of slowdown
you are seeing" quick-and-dirty patch.

Speaking of the checks in there - if anything, might_fault() in those
suckers belongs outside of the loop; note that even on the kmap_atomic()
side of copy_page_to_iter_iovec() we do stuff like fault_in_pages_writeable().

BTW, ..._inatomic is a very unfortunate name, IMO - it's *not* safe
to use in atomic contexts as-is, to start with; the caller needs to take
care of pagefault_disable().  If anything, __copy_from_user_nofault() would
probably be better...

I really wonder about the low dispersion in those tests - IME on amd64
boxen it tends to be ~5% or so; what's normal for arm?