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

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Date:   Fri, 10 Mar 2023 18:21:07 -0500
From:   "Theodore Ts'o" <tytso@....edu>
To:     Yangtao Li <frank.li@...o.com>
Cc:     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.

All aside from the arguments over performance, I'm not at *all*
convinced by the "it's more readable" argument.

So yeah, let's not.  We have i_blkbits for a reason, and it's because
shifting right is just simpler and easier.

BTW, doing a 64-bit division on a 32-bit platforms causes compile
failures, which was the cause of the test bot complaint:

   ld: fs/ext4/inode.o: in function `mpage_process_page_bufs':
>> inode.c:(.text+0xbda): undefined reference to `__divdi3'

On 32-bit platforms --- i386 in particular --- the 64-bit division
results in an out-of-line call to a helper function that is not
supplied in the kernel compilation environment, so not only is it
slower, it Just Doesn't Work.

						- Ted