linux-kernel - Re: [rfc 0/2] Introducing VmFlags field into smaps output

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Message-Id: <20121022233025.09ec2d92.akpm@linux-foundation.org>
Date:	Mon, 22 Oct 2012 23:30:25 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	Pavel Emelyanov <xemul@...allels.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [rfc 0/2] Introducing VmFlags field into smaps output

On Tue, 23 Oct 2012 10:13:41 +0400 Cyrill Gorcunov <gorcunov@...nvz.org> wrote:

> On Mon, Oct 22, 2012 at 02:51:58PM -0700, Andrew Morton wrote:
> ...
> > > +	__seq_show_vmflag(VM_DONTEXPAND,"de ");
> > > +	__seq_show_vmflag(VM_ACCOUNT,	"ac ");
> > > +	__seq_show_vmflag(VM_NORESERVE,	"nr ");
> > > +	__seq_show_vmflag(VM_HUGETLB,	"ht ");
> > > +	__seq_show_vmflag(VM_NONLINEAR,	"nl ");
> > > +	__seq_show_vmflag(VM_ARCH_1,	"ar ");
> > > +	__seq_show_vmflag(VM_DONTDUMP,	"dd ");
> > > +	__seq_show_vmflag(VM_MIXEDMAP,	"mm ");
> > > +	__seq_show_vmflag(VM_HUGEPAGE,	"hg ");
> > > +	__seq_show_vmflag(VM_NOHUGEPAGE,"nh ");
> ...
> > 
> > This code would be much denser if we did the old
> > 
> > 	static const char foo[] = "rdwrexshmrmw...";
> > 	unsigned i;
> > 
> > 	for (i = 0; i < BITS_PER_LONG; i++) {
> > 		if (flags & (1 << i))
> > 			seq_printf("%c%c ", foo[i * 2], foo[i * 2 + 1]);
> > 	}
> > 
> > trick.  But then we'd have to remember to fix foo[] each time we
> > alter the flags, so maybe let's not do that.
> 
> Yup, but not only that, this kind of trick hides associativity between
> VM_ constant and mnemonic, so on changes one would have to figure out
> which position some flag has in this foo[] array, so I vote for not
> use it :-)

Well you could do

struct {
	char x[2];
} y[] = {
	[CLOG2(VM_DONTEXPAND)] =	{ 'd', 'e' },
	[CLOG2(VM_ACCOUNT)] =		{ 'a', 'c' },
	[CLOG2(VM_NORESERVE)] =		{ 'n', 'r' },
};

	...

	for (i = 0; i < BITS_PER_LONG; i++) {
		if (flags & (1 << i))
			seq_printf("%c%c ", y[i][0], y[i][1]);
	}

where CLOG2() is extracted from the guts of ilog2().

I'll stop now :)
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