linux-kernel - Re: [PATCH v2 06/10] mm/swap: use npage_to_sectors() and PAGE

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Date:   Thu, 14 May 2020 21:06:47 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Minchan Kim <minchan@...nel.org>, Nitin Gupta <ngupta@...are.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Jens Axboe <axboe@...nel.dk>, Coly Li <colyli@...e.de>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        linux-block <linux-block@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>, dm-devel <dm-devel@...hat.com>,
        Song Liu <song@...nel.org>,
        linux-raid <linux-raid@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 06/10] mm/swap: use npage_to_sectors() and
 PAGE_SECTORS to clean up code

On Thu, May 07, 2020 at 03:50:56PM +0800, Zhen Lei wrote:
> @@ -266,7 +266,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
>  
>  static sector_t swap_page_sector(struct page *page)
>  {
> -	return (sector_t)__page_file_index(page) << (PAGE_SHIFT - 9);
> +	return npage_to_sectors((sector_t)__page_file_index(page));

If you make npage_to_sectors() a proper function instead of a macro,
you can do the casting inside the function instead of in the callers
(which is prone to bugs).

Also, this is a great example of why page_to_sector() was a better name
than npage_to_sectors().  This function doesn't return a count of sectors,
it returns a sector number within the swap device.