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Date:   Mon, 18 May 2020 22:12:35 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Cc:     viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, david@fromorbit.com,
        hch@infradead.org, willy@infradead.org
Subject: Re: [PATCH 10/10] mm/migrate.c: call detach_page_private to cleanup
 code
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        <20200517214718.468-11-guoqing.jiang@cloud.ionos.com>
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On Sun, 17 May 2020 23:47:18 +0200 Guoqing Jiang <guoqing.jiang@cloud.ionos.com> wrote:

> We can cleanup code a little by call detach_page_private here.
> 
> ...
>
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -804,10 +804,7 @@ static int __buffer_migrate_page(struct address_space *mapping,
>  	if (rc != MIGRATEPAGE_SUCCESS)
>  		goto unlock_buffers;
>  
> -	ClearPagePrivate(page);
> -	set_page_private(newpage, page_private(page));
> -	set_page_private(page, 0);
> -	put_page(page);
> +	set_page_private(newpage, detach_page_private(page));
>  	get_page(newpage);
>  
>  	bh = head;

mm/migrate.c: In function '__buffer_migrate_page':
./include/linux/mm_types.h:243:52: warning: assignment makes integer from pointer without a cast [-Wint-conversion]
 #define set_page_private(page, v) ((page)->private = (v))
                                                    ^
mm/migrate.c:800:2: note: in expansion of macro 'set_page_private'
  set_page_private(newpage, detach_page_private(page));
  ^~~~~~~~~~~~~~~~

The fact that set_page_private(detach_page_private()) generates a type
mismatch warning seems deeply wrong, surely.

Please let's get the types sorted out - either unsigned long or void *,
not half-one and half-the other.  Whatever needs the least typecasting
at callsites, I suggest.

And can we please implement set_page_private() and page_private() with
inlined C code?  There is no need for these to be macros.