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

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Date:   Fri, 10 Mar 2023 06:46:12 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Yangtao Li <frank.li@...o.com>, tytso@....edu,
        adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: convert to DIV_ROUND_UP() in
 mpage_process_page_bufs()

On Thu, Mar 09, 2023 at 10:43:55PM -0800, Eric Biggers wrote:
> On Fri, Mar 10, 2023 at 06:37:29AM +0000, Al Viro wrote:
> > On Thu, Mar 09, 2023 at 10:17:16PM -0800, Eric Biggers wrote:
> > > 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.
> > 
> > Which compiler is doing that?
> 
> $ gcc --version
> gcc (GCC) 12.2.1 20230201
> 
> i_blocksize(inode) is not a constant, so this should not be particularly
> surprising.  One might hope that a / (1 << b) would be optimized into a >> b,
> but that doesn't seem to happen.

It really ought to be a / (1u << b), though...