linux-kernel - Re: [PATCH] NOMMU: Separate out VMAs

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Date:	Tue, 07 Aug 2007 15:37:20 +0200
From:	Bernd Schmidt <bernds_cb1@...nline.de>
To:	David Howells <dhowells@...hat.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Wu, Bryan" <Bryan.Wu@...log.com>
Subject: Re: [PATCH] NOMMU: Separate out VMAs

David Howells wrote:

> +	/* we allocated a power-of-2 sized page set, so we need to trim off the
> +	 * excess */
> +	total = 1 << order;
> +	atomic_add(total, &mmap_pages_allocated);
> +
> +	point = len >> PAGE_SHIFT;
> +	while (point < total) {
> +		order = ilog2(total - point);
> +		_debug("shave %u/%lu", 1 << order, total - point);
> +		atomic_sub(1 << order, &mmap_pages_allocated);
> +		__free_pages(pages + point, order);
> +		point += 1 << order;
> +	}

Note that we've had a similar change in our kernel, and we've had to
revert it since the effect on fragmentation is just horrendous.  Without
this, we could run a complete gcc testsuite on the board; with it we'd
run out of high-order pages somewhere in the middle.

This probably wants to be dependent on something like MAP_TRIM_EXCESS.


Bernd
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