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

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Date:   Fri, 12 Jun 2020 15:55:51 +0530
From:   afzal mohammed <afzal.mohd.ma@...il.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        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,

On Wed, Jun 10, 2020 at 12:10:21PM +0200, Linus Walleij wrote:
> On Mon, Jun 8, 2020 at 1:09 PM afzal mohammed <afzal.mohd.ma@...il.com> wrote:

> > 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.
> 
> I am happy to help!

Thanks Linus

> I am anyway working on MMU-related code (KASan) so I need to be on
> top of this stuff.

i earlier went thr' KASAN series secretly & did learn a thing or two
from that!

> What test is appropriate for this? I would intuitively think hackbench?

'dd', i think, as you mentioned 'hackbench' i will use that as well.

> > 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
> 
> I will be very excited to look at patches or a git branch once you have
> something you want to show. Also to just understand how you go about
> this.

Don't put too much expectation on me, this is more of a learning for
me. For user copy, the baby steps has been posted (To'ed you). On the
static kernel mapping on vmalloc front, i do not want to post the
patches in the current shape, though git-ized, will result in me
getting mercilessly thrashed in public :). Many of the other platforms
would fail and is not multi-platform friendly. i do not yet have a
public git branch, i can send you the (ugly) patches separately, just
let me know.

> I have several elder systems under my roof

i have only a few low RAM & CPU systems, so that is certainly helpful.

> so my contribution could hopefully be to help and debug any issues

If you would like, we can work together, at the same time keep in mind
that me spending time on it would be intermittent & erratic (though i
am trying to keep a consistent, but slow pace) perhaps making it
difficult to coordinate. Or else i will continue the same way & request
your help when required.

For the next 3 weeks, right now, i cannot say whether i would be able
to spend time on it, perhaps might be possible, but only during that
time i will know.

Regards
afzal