Date: Mon, 12 Aug 2013 15:37:25 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, xemul@parallels.com, mpm@selenic.com,
        xiaoguangrong@linux.vnet.ibm.com, mtosatti@redhat.com,
        kosaki.motohiro@gmail.com, sfr@canb.auug.org.au, peterz@infradead.org,
        aneesh.kumar@linux.vnet.ibm.com, Ingo Molnar <mingo@elte.hu>,
        "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [patch 2/2] [PATCH] mm: Save soft-dirty bits on file pages
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On Mon, 12 Aug 2013 15:28:06 -0700 Andy Lutomirski <luto@amacapital.net> wrote:

> > +#define _mfrob(v,r,m,l)                ((((v) >> (r)) & (m)) << (l))
> > +#define __frob(v,r,l)          (((v) >> (r)) << (l))
> > +
> >  #ifdef CONFIG_MEM_SOFT_DIRTY
> >
> 
> If I'm understanding this right, the idea is to take the bits in the
> range a..b of v and stick them at c..d, where a-b == c-d.  Would it
> make sense to change this to look something like
> 
> #define __frob(v, inmsb, inlsb, outlsb) ((v >> inlsb) & ((1<<(inmsb -
> inlsb + 1)-1) << outlsb)
> 
> For extra fun, there could be an __unfrob macro that takes the same
> inmsg, inlsb, outlsb parameters but undoes it so that it's (more)
> clear that the operations that are supposed to be inverses are indeed
> inverses.

hm, I seem to remember writing
drivers/net/ethernet/3com/3c59x.c:BFINS() and BFEXT() shortly after the
invention of the electronic computer.

I'm kinda surprised that we don't already have something like this in
kernel.h or somewhere - there's surely a ton of code which does such
things.

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