linux-kernel - Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap

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Date:   Sat, 13 Jun 2020 16:31:02 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     afzal mohammed <afzal.mohd.ma@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Linus Walleij <linus.walleij@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Nicolas Pitre <nico@...xnic.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic()

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.