linux-kernel - Re: New helper to free highmem pages in larger chunks

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Date:	Mon, 5 Oct 2015 15:09:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	Robin Holt <robin.m.holt@...il.com>,
	Nathan Zimmer <nzimmer@....com>, Jiang Liu <liuj97@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>, Mel Gorman <mgorman@...e.de>
Subject: Re: New helper to free highmem pages in larger chunks

On Sat, 3 Oct 2015 18:25:13 +0530 Vineet Gupta <Vineet.Gupta1@...opsys.com> wrote:

> Hi,
> 
> I noticed increased boot time when enabling highmem for ARC. Turns out that
> freeing highmem pages into buddy allocator is done page at a time, while it is
> batched for low mem pages. Below is call flow.
> 
> I'm thinking of writing free_highmem_pages() which takes start and end pfn and
> want to solicit some ideas whether to write it from scratch or preferably call
> existing __free_pages_memory() to reuse the logic to convert a pfn range into
> {pfn, order} tuples.
> 
> For latter however there are semantical differences as you can see below which I'm
> not sure of:
>   -highmem page->count is set to 1, while 0 for low mem

That would be weird.

Look more closely at __free_pages_boot_core() - it uses
set_page_refcounted() to set the page's refcount to 1.  Those
set_page_count() calls look superfluous to me.

>   -atomic clearing of page reserved flag vs. non atomic

I doubt if the atomic is needed - who else can be looking at this page
at this time?


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