linux-kernel - Re: [PATCH 3/6] writeback: avoid unnecessary calculation of bdi dirty thresholds

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Message-ID: <20100720033437.GE6087@localhost>
Date:	Tue, 20 Jul 2010 11:34:37 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/6] writeback: avoid unnecessary calculation of bdi
 dirty thresholds

On Tue, Jul 20, 2010 at 05:35:20AM +0800, Andrew Morton wrote:
> On Sun, 11 Jul 2010 10:06:59 +0800
> Wu Fengguang <fengguang.wu@...el.com> wrote:
> 
> > Split get_dirty_limits() into global_dirty_limits()+bdi_dirty_limit(),
> > so that the latter can be avoided when under global dirty background
> > threshold (which is the normal state for most systems).
> > 
> 
> mm/page-writeback.c: In function 'balance_dirty_pages_ratelimited_nr':
> mm/page-writeback.c:466: warning: 'dirty_exceeded' may be used uninitialized in this function
> 
> This was a real bug.

Thanks! But how do you catch this? There are no warnings in my compile test.

I noticed that there is a gcc option "-Wuninitialized", which will be turned on
with "-Wall" and "-O2" as used in the following command:

  gcc -Wp,-MD,mm/.page-writeback.o.d  -nostdinc -isystem /usr/lib/gcc/x86_64-linux-gnu/4.4.4/include -I/cc/linux-2.6.33/arch/x86/include -Iinclude  -include include/generated/autoconf.h -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -O2 -m64 -march=core2 -mno-red-zone -mcmodel=kernel -funit-at-a-time -maccumulate-outgoing-args -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -Wframe-larger-than=2048 -fno-stack-protector -fno-omit-frame-pointer -fno-optimize-sibling-calls -g -pg -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fno-dwarf2-cfi-asm -fconserve-stack   -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(page_writeback)"  -D"KBUILD_MODNAME=KBUILD_STR(page_writeback)"  -c -o mm/page-writeback.o mm/page-writeback.c


My gcc version is 

$ gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.4.4-6' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --ena
ble-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --with-arch-32=i586 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu                                            Thread model: posix
gcc version 4.4.4 (Debian 4.4.4-6) 

Thanks,
Fengguang

> --- a/mm/page-writeback.c~writeback-avoid-unnecessary-calculation-of-bdi-dirty-thresholds-fix
> +++ a/mm/page-writeback.c
> @@ -463,7 +463,7 @@ static void balance_dirty_pages(struct a
>  	unsigned long bdi_thresh;
>  	unsigned long pages_written = 0;
>  	unsigned long pause = 1;
> -	int dirty_exceeded;
> +	bool dirty_exceeded = false;
>  	struct backing_dev_info *bdi = mapping->backing_dev_info;
>  
>  	for (;;) {
> _
--
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