linux-kernel - Re: [patch 2/2] [PATCH] mm: Save soft-dirty bits on file pages

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Date:	Mon, 12 Aug 2013 15:37:25 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, xemul@...allels.com, mpm@...enic.com,
	xiaoguangrong@...ux.vnet.ibm.com, mtosatti@...hat.com,
	kosaki.motohiro@...il.com, sfr@...b.auug.org.au,
	peterz@...radead.org, aneesh.kumar@...ux.vnet.ibm.com,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 2/2] [PATCH] mm: Save soft-dirty bits on file pages

On Mon, 12 Aug 2013 15:28:06 -0700 Andy Lutomirski <luto@...capital.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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