linux-kernel - Re: [PATCH] ext4: convert to DIV_ROUND_UP() in mpage_process_page

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Message-ID: <ZArLbO1ckmcXwQf0@sol.localdomain>
Date:   Thu, 9 Mar 2023 22:17:16 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Yangtao Li <frank.li@...o.com>
Cc:     tytso@....edu, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        viro@...iv.linux.org.uk
Subject: Re: [PATCH] ext4: convert to DIV_ROUND_UP() in
 mpage_process_page_bufs()

On Fri, Mar 10, 2023 at 02:07:34PM +0800, Yangtao Li wrote:
> Just for better readability, no code logic change.
> 
> Signed-off-by: Yangtao Li <frank.li@...o.com>
> ---
>  fs/ext4/inode.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index d251d705c276..d121cde74522 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -2218,8 +2218,7 @@ static int mpage_process_page_bufs(struct mpage_da_data *mpd,
>  {
>  	struct inode *inode = mpd->inode;
>  	int err;
> -	ext4_lblk_t blocks = (i_size_read(inode) + i_blocksize(inode) - 1)
> -							>> inode->i_blkbits;
> +	ext4_lblk_t blocks = DIV_ROUND_UP(i_size_read(inode), i_blocksize(inode));
>  

Please don't do this.  This makes the code compile down to a division, which is
far less efficient.  I've verified this by checking the assembly generated.

- Eric