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Date:   Sat, 13 Jun 2020 16:31:02 +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>,
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        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()
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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.