Re: [PATCH] fs/buffer.c: Optimize grow_buffer function.

[Sean Fu]

On Sat, Jul 21, 2018 at 07:21:16PM +0100, Al Viro wrote:
> 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@linux-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@linux-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?

Got it, Thanks
This size can be page size(4096) besides block size 
(1<<bdev->bd_inode->i_blkbits).

I have another question about __getblk_slow function in fs/buffer.c
Actually __find_get_block already was invoked before entering __getblk_slow.
Can we move __find_get_block after grow_buffers in for loop of __getblk_slow?

for (;;) {
        ...
        ret = grow_buffers(bdev, block, size, gfp);
        ...
        bh = __find_get_block(bdev, block, size);
        ...
}