linux-ext4 - Re: [PATCH v3 4/6] ext4: convert to use i

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Message-ID: <20230310033213.GG3390869@ZenIV>
Date:   Fri, 10 Mar 2023 03:32:13 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Yangtao Li <frank.li@...o.com>
Cc:     xiang@...nel.org, chao@...nel.org, huyue2@...lpad.com,
        jefflexu@...ux.alibaba.com, tytso@....edu,
        adilger.kernel@...ger.ca, rpeterso@...hat.com, agruenba@...hat.com,
        mark@...heh.com, jlbec@...lplan.org, joseph.qi@...ux.alibaba.com,
        brauner@...nel.org, linux-erofs@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
        cluster-devel@...hat.com, ocfs2-devel@....oracle.com,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v3 4/6] ext4: convert to use i_blockmask()

On Fri, Mar 10, 2023 at 03:19:40AM +0000, Al Viro wrote:
> On Thu, Mar 09, 2023 at 11:21:25PM +0800, Yangtao Li wrote:
> > Use i_blockmask() to simplify code.
> > 
> > Signed-off-by: Yangtao Li <frank.li@...o.com>
> > ---
> > v3:
> > -none
> > v2:
> > -convert to i_blockmask()
> >  fs/ext4/inode.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index d251d705c276..eec36520e5e9 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -2218,7 +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)
> > +	ext4_lblk_t blocks = (i_size_read(inode) + i_blockmask(inode))
> >  							>> inode->i_blkbits;
> 
> Umm...  That actually asks for DIV_ROUND_UP(i_size_read_inode(), i_blocksize(inode)) -
> compiler should bloody well be able to figure out that division by (1 << n) is
> shift down by n and it's easier to follow that way...

BTW, here the fact that you are adding the mask is misleading - it's true,
but we are not using it as a mask - it's straight arithmetics.

One can do an equivalent code where it would've been used as a mask, but that
would be harder to read -
	(((i_size_read(inode) - 1) | i_blockmask(inode)) + 1) >> inode->i_blkbits
and I doubt anyone wants that kind of puzzles to stumble upon.