linux-kernel - Re: [PATCH] fs/buffer.c: Optimize grow

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Message-ID: <20180721182116.GC30522@ZenIV.linux.org.uk>
Date:   Sat, 21 Jul 2018 19:21:16 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Sean Fu <fxinrong@...il.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/buffer.c: Optimize grow_buffer function.

On Sun, Jul 22, 2018 at 01:30:17AM +0800, Sean Fu wrote:
> Use PAGE_SHIFT and i_blkbits of bd_inode directly to avoid ugly sizebits
> calculation.
> Remove ugly sizebits calculation.
> Remove unnecessary sizebits parameter of grow_dev_page.
> 
> Reduces code size:
> 
> Before:
> 
> sean@...ux-zmni:~/sda5/source/linus_repo/linux> size fs/buffer.o
>    text	   data	    bss	    dec	    hex	filename
>   34037	   1510	     16	  35563	   8aeb	fs/buffer.o
> 
> After:
> 
> sean@...ux-zmni:~/sda5/source/linus_repo/linux> size fs/buffer.o
>    text	   data	    bss	    dec	    hex	filename
>   34021	   1510	     16	  35547	   8adb	fs/buffer.o

First of all, 16 bytes is pretty much noise.  What's more, the "remove ugly
sizebits calculation" part really needs an explanation of the reasons why
it's safe.  You assume that size == 1<<bdev->bd_inode->i_blkbits; where's
the proof that it always holds?