linux-kernel - Re: [PATCH 5/5] proc: export more page flags in /proc/kpageflags

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Date:	Tue, 28 Apr 2009 19:55:27 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"mpm@...enic.com" <mpm@...enic.com>,
	"adobriyan@...il.com" <adobriyan@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Chandra Seetharaman <sekharan@...ibm.com>,
	Nathan Lynch <ntl@...ox.com>, Olof Johansson <olof@...om.net>,
	Helge Deller <deller@...isc-linux.org>
Subject: Re: [PATCH 5/5] proc: export more page flags in /proc/kpageflags

On Wed, 29 Apr 2009 10:38:42 +0800 Wu Fengguang <fengguang.wu@...el.com> wrote:

> > > +#define kpf_copy_bit(uflags, kflags, visible, ubit, kbit)		\
> > > +	do {								\
> > > +		if (visible || genuine_linus())				\
> > > +			uflags |= ((kflags >> kbit) & 1) << ubit;	\
> > > +	} while (0);
> > 
> > Did this have to be implemented as a macro?
> > 
> > It's bad, because it might or might not reference its argument, so if
> > someone passes it an expression-with-side-effects, the end result is
> > unpredictable.  A C function is almost always preferable if possible.
> 
> Just tried inline function, the code size is increased slightly:
> 
>           text   data    bss     dec    hex   filename
> macro     1804    128      0    1932    78c   fs/proc/page.o
> inline    1828    128      0    1956    7a4   fs/proc/page.o
> 

hm, I wonder why.  Maybe it fixed a bug ;)

The code is effectively doing

	if (expr1)
		something();
	if (expr1)
		something_else();
	if (expr1)
		something_else2();

etc.  Obviously we _hope_ that the compiler turns that into

	if (expr1) {
		something();
		something_else();
		something_else2();
	}

for us, but it would be good to check...
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