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Date:   Sun, 15 Aug 2021 16:30:49 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Mianhan Liu <liumh1@shanghaitech.edu.cn>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Eric Dumazet <edumazet@google.com>
Subject: Re: [PATCH -next v2] include/linux/mm.h: move nr_free_buffer_pages
 from swap.h to mm.h
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References: <20210815115057.1511-1-liumh1@shanghaitech.edu.cn>
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On Sun, Aug 15, 2021 at 07:50:57PM +0800, Mianhan Liu wrote:
> nr_free_buffer_pages could be exposed through mm.h instead of swap.h.
> The advantage of this change is that it can reduce the obsolete includes.
> For example, net/ipv4/tcp.c wouldn't need swap.h any more since it has
> already included mm.h.
> Moreover, after preprocessing all the files that use nr_free_buffer_pages,
> it turns out that those files have already included mm.h.
> Thus, we can move nr_free_buffer_pages from swap.h to mm.h safely. This change
> will not affect the compilation of other files.

In general, we're trying to move in the other direction; move things out
of mm.h and reduce the number of files which include mm.h.  That said,
I don't see what nr_free_buffer_pages() has to do with the rest of
swap.h, so I'm OK with moving this over.

That said, there are a number of other files which should have swap.h
removed.  eg:

drivers/mmc/core/mmc_test.c:#include <linux/swap.h>             /* For nr_free_buffer_pages() */

and it would be good to check the other files which use
nr_free_buffer_pages() to see if they can have swap.h removed too.

> +++ b/include/linux/mm.h
> @@ -871,6 +871,7 @@ void put_pages_list(struct list_head *pages);
>  void split_page(struct page *page, unsigned int order);
>  void folio_copy(struct folio *dst, struct folio *src);
>  
> +extern unsigned long nr_free_buffer_pages(void);

Please remove the 'extern' while you're moving this.

> +++ b/net/ipv4/tcp.c

If you're interested in cleaning up the headers, it'd be nice if less
of the networking code depended on pagemap.h ...