linux-kernel - Re: [PATCH 07/13] mm: meminit: Initialise a subset of struct pages if CONFIG_DEFERRED_STRUCT_PAGE

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Date:	Wed, 29 Apr 2015 14:19:01 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Nathan Zimmer <nzimmer@....com>,
	Dave Hansen <dave.hansen@...el.com>,
	Waiman Long <waiman.long@...com>,
	Scott Norton <scott.norton@...com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 07/13] mm: meminit: Initialise a subset of struct pages
 if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set

On Tue, 28 Apr 2015 15:37:04 +0100 Mel Gorman <mgorman@...e.de> wrote:

> +/*
> + * Deferred struct page initialisation requires some early init functions that
> + * are removed before kswapd is up and running. The feature depends on memory
> + * hotplug so put the data and code required by deferred initialisation into
> + * the __meminit section where they are preserved.
> + */
> +#ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT
> +#define __defermem_init __meminit
> +#define __defer_init    __meminit
> +#else
> +#define __defermem_init
> +#define __defer_init __init
> +#endif

I still don't get it :(

__defermem_init:

	if (CONFIG_DEFERRED_STRUCT_PAGE_INIT) {
		if (CONFIG_MEMORY_HOTPLUG)
			retain
	} else {
		retain
	}

    but CONFIG_DEFERRED_STRUCT_PAGE_INIT depends on
    CONFIG_MEMORY_HOTPLUG, so this becomes

	if (CONFIG_DEFERRED_STRUCT_PAGE_INIT) {
		retain
	} else {
		retain
	}

    which becomes

	retain

    so why does __defermem_init exist?



__defer_init:

	if (CONFIG_DEFERRED_STRUCT_PAGE_INIT) {
		if (CONFIG_MEMORY_HOTPLUG)
			retain
	} else {
		discard
	}

    becomes

	if (CONFIG_DEFERRED_STRUCT_PAGE_INIT) {
		retain
	} else {
		discard
	}

    this one makes sense, but could be documented much more clearly!


And why does the comment refer to "and data".  There is no
__defer_initdata, etc.  Just not needed yet?

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