Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
Subject: Re: [PATCH] mm: change "char *bdi_unknown_name" to "char
 bdi_unknown_name[]"
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20220512082637.24649-1-liqiong@nfschina.com>
 <20220512130051.94a0c53e5d1498292473975d@linux-foundation.org>
From:   liqiong <liqiong@nfschina.com>
Message-ID: <2d698914-befd-0283-71ed-599aa1bf9e3d@nfschina.com>
Date:   Fri, 13 May 2022 14:34:56 +0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.2.1
MIME-Version: 1.0
In-Reply-To: <20220512130051.94a0c53e5d1498292473975d@linux-foundation.org>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Content-Language: en-US
Precedence: bulk

在 2022年05月13日 04:00, Andrew Morton 写道:
> On Thu, 12 May 2022 16:26:37 +0800 liqiong <liqiong@nfschina.com> wrote:
>
>> "char bdi_unknown_nam[]" string form declares a single variable.
>> It is better then "char *bdi_unknown_name" which creates two
>> variables.
>>
>> ...
>>
>> --- a/mm/backing-dev.c
>> +++ b/mm/backing-dev.c
>> @@ -20,7 +20,7 @@ struct backing_dev_info noop_backing_dev_info;
>>  EXPORT_SYMBOL_GPL(noop_backing_dev_info);
>>  
>>  static struct class *bdi_class;
>> -static const char *bdi_unknown_name = "(unknown)";
>> +static const char bdi_unknown_name[] = "(unknown)";
>>  
> heh, fun patch.  We actually do this quite a lot.  
>
> 	grep -r "^[a-z].*char \*[a-z].*= \"" .
>
> is a pathetic pattern which catches a lot of them.
>
>
> However.  I expected your patch to shrink the kernel a bit, but it has
> the opposite effect:
>
> hp2:/usr/src/25> size mm/backing-dev.o
>    text	   data	    bss	    dec	    hex	filename
>   21288	   9396	   3808	  34492	   86bc	mm/backing-dev.o-before
>   21300	   9428	   3808	  34536	   86e8	mm/backing-dev.o-after
>
> Even .data became larger.  I didn't investigate why.
Hi,
It seems  the patch creates a new section:
0000000000000000 l       ___ksymtab_gpl+bdi_dev_name    0000000000000000 __ksymtab_bdi_dev_name
0000000000000f60 l     O .rodata        000000000000000a bdi_unknown_name
And  put  "bdi_unknown_name"  at  .rodata.str1.1:

This work was published in "KernelJanitors/Todo". It says:

"foo []" is better because it declares a single variable,   For variables marked __initdata, the "*foo" form causes
only the pointer, not the string itself, to be dropped from the kernel image, which is a bug.
Using the "foo[]" form with regular 'ole local variables also makes the assembly shorter.

I thought it make sense,  so i  searched the mm tree by "grep  -nHre char.*\*.*=.*\"",  and checked all the "char *foo"
string form,  It seems only this one should be fixed.