linux-kernel - Re: ARM: vmsplit 4g/4g

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Date:   Mon, 8 Jun 2020 16:39:02 +0530
From:   afzal mohammed <afzal.mohd.ma@...il.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: ARM: vmsplit 4g/4g

Hi,

[ my previous mail did not make into linux-arm-kernel mailing list,
 got a  mail saying it has a suspicious header and that it is waiting
 moderator approval ]

On Sun, Jun 07, 2020 at 05:11:16PM +0100, Russell King - ARM Linux admin wrote:
> On Sun, Jun 07, 2020 at 06:29:32PM +0530, afzal mohammed wrote:

> > get_user_pages_fast() followed by kmap_atomic() & then memcpy() seems
> > to work in principle for user copy.
> 
> Have you done any performance evaluation of the changes yet? I think
> it would be a good idea to keep that in the picture. If there's any
> significant regression, then that will need addressing.

Not yet. Yes, i will do the performance evaluation.

i am also worried about the impact on performance as these
[ get_user_pages() or friends, kmap_atomic() ] are additionally
invoked in the copy_{from,to}_user() path now.

Note that this was done on a topic branch for user copy. Changes for
kernel static mapping to vmalloc has not been merged with these.
Also having kernel lowmem w/ a separate asid & switching at kernel
entry/exit b/n user & kernel lowmem by changing ttbr0 is yet to be
done. Quite a few things remaining to be done to achieve vmsplit 4g/4g

Regards
afzal