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Message-ID: <20200613154118.GH23230@ZenIV.linux.org.uk>
Date: Sat, 13 Jun 2020 16:41:18 +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 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.
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