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

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Message-Id: <20230310062738.69663-1-frank.li@vivo.com>
Date:   Fri, 10 Mar 2023 14:27:38 +0800
From:   Yangtao Li <frank.li@...o.com>
To:     tytso@....edu, adilger.kernel@...ger.ca
Cc:     linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        viro@...iv.linux.org.uk, hsiangkao@...ux.alibaba.com,
        linux-erofs@...ts.ozlabs.org
Subject: Re: [PATCH] ext4: convert to DIV_ROUND_UP() in mpage_process_page_bufs()

> 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.

How much is the performance impact? So should the following be modified as shift operations as well?

fs/erofs/namei.c:92:    int head = 0, back = DIV_ROUND_UP(dir->i_size, EROFS_BLKSIZ) - 1;
fs/erofs/zmap.c:252:    const unsigned int totalidx = DIV_ROUND_UP(inode->i_size, EROFS_BLKSIZ);
fs/erofs/decompressor.c:14:#define LZ4_MAX_DISTANCE_PAGES       (DIV_ROUND_UP(LZ4_DISTANCE_MAX, PAGE_SIZE) + 1)
fs/erofs/decompressor.c:56:                                     DIV_ROUND_UP(distance, PAGE_SIZE) + 1 :
fs/erofs/decompressor.c:70:     unsigned long bounced[DIV_ROUND_UP(LZ4_MAX_DISTANCE_PAGES,
fs/erofs/data.c:84:     nblocks = DIV_ROUND_UP(inode->i_size, EROFS_BLKSIZ);

Thx,
Yangtao