Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Sat, 13 Jun 2020 16:41:18 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     afzal mohammed <afzal.mohd.ma@gmail.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Russell King - ARM Linux admin <linux@armlinux.org.uk>,
        Linus Walleij <linus.walleij@linaro.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Nicolas Pitre <nico@fluxnic.net>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic()
Message-ID: <20200613154118.GH23230@ZenIV.linux.org.uk>
References: <cover.1591885760.git.afzal.mohd.ma@gmail.com>
 <9e1de19f35e2d5e1d115c9ec3b7c3284b4a4e077.1591885760.git.afzal.mohd.ma@gmail.com>
 <CAK8P3a1XUJHC0kG_Qwh4D4AoxTgCL5ggHd=45yNSmzaYWLUWXw@mail.gmail.com>
 <20200612135538.GA13399@afzalpc>
 <CAK8P3a25ffh_2Y1xKDbkL2xU9nLpGbEq7j6xHdODEwUtavgdwA@mail.gmail.com>
 <20200613120432.GA5319@afzalpc>
 <20200613125126.GE23230@ZenIV.linux.org.uk>
 <20200613125615.GF23230@ZenIV.linux.org.uk>
 <20200613134236.GA4086@afzalpc>
 <20200613153102.GG23230@ZenIV.linux.org.uk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200613153102.GG23230@ZenIV.linux.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Sat, Jun 13, 2020 at 04:31:02PM +0100, Al Viro wrote:
> On Sat, Jun 13, 2020 at 07:12:36PM +0530, afzal mohammed wrote:
> > Hi,
> > 
> > On Sat, Jun 13, 2020 at 01:56:15PM +0100, Al Viro wrote:
> > 
> > > Incidentally, what about get_user()/put_user()?  _That_ is where it's
> > > going to really hurt...
> > 
> > All other uaccess routines are also planned to be added, posting only
> > copy_{from,to}_user() was to get early feedback (mentioned in the
> > cover letter)
> 
> Sure, but what I mean is that I'd expect the performance loss to be
> dominated by that, not by copy_from_user/copy_to_user on large amounts
> of data.  Especially on the loads like kernel builds - a lot of stat()
> and getdents() calls there.

To clarify: stat() means small copy_to_user(), getdents() - a mix of
put_user() and small copy_to_user().  I would be very surprised if it
does not hurt a lot.