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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>,
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        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>
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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.