Date: Thu, 30 Mar 2017 19:48:24 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Russell King - ARM Linux <linux@armlinux.org.uk>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Richard Henderson <rth@twiddle.net>, Will Deacon <will.deacon@arm.com>,
        Haavard Skinnemoen <hskinnemoen@gmail.com>,
        Vineet Gupta <vgupta@synopsys.com>, Steven Miao <realmz6@gmail.com>,
        Jesper Nilsson <jesper.nilsson@axis.com>,
        Mark Salter <msalter@redhat.com>,
        Yoshinori Sato <ysato@users.sourceforge.jp>,
        Richard Kuo <rkuo@codeaurora.org>, Tony Luck <tony.luck@intel.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        James Hogan <james.hogan@imgtec.com>, Michal Simek <monstr@monstr.eu>,
        David Howells <dhowells@redhat.com>, Ley Foon Tan <lftan@altera.com>,
        Jonas Bonn <jonas@southpole.se>, Helge Deller <deller@gmx.de>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Chen Liqin <liqin.linux@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Chris Metcalf <cmetcalf@mellanox.com>,
        Richard Weinberger <richard@nod.at>, Guan Xuetao <gxt@mprc.pku.edu.cn>,
        Thomas Gleixner <tglx@linutronix.de>, Chris Zankel <chris@zankel.net>
Subject: Re: [RFC][CFT][PATCHSET v1] uaccess unification
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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?