linux-kernel - Re: [RFC 0/4] Transparent on-demand struct page initialization embedded in the buddy allocator

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Message-ID: <20130715150040.GA3421@sgi.com>
Date:	Mon, 15 Jul 2013 10:00:40 -0500
From:	Robin Holt <holt@....com>
To:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>
Cc:	Robin Holt <holt@....com>, Nate Zimmer <nzimmer@....com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>, Rob Landley <rob@...dley.net>,
	Mike Travis <travis@....com>,
	Daniel J Blueman <daniel@...ascale-asia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Yinghai Lu <yinghai@...nel.org>, Mel Gorman <mgorman@...e.de>
Subject: Re: [RFC 0/4] Transparent on-demand struct page initialization
 embedded in the buddy allocator

On Thu, Jul 11, 2013 at 09:03:51PM -0500, Robin Holt wrote:
> We have been working on this since we returned from shutdown and have
> something to discuss now.  We restricted ourselves to 2MiB initialization
> to keep the patch set a little smaller and more clear.
> 
> First, I think I want to propose getting rid of the page flag.  If I knew
> of a concrete way to determine that the page has not been initialized,
> this patch series would look different.  If there is no definitive
> way to determine that the struct page has been initialized aside from
> checking the entire page struct is zero, then I think I would suggest
> we change the page flag to indicate the page has been initialized.

Ingo or HPA,

Did I implement this wrong or is there a way to get rid of the page flag
which is not going to impact normal operation?  I don't want to put too
much more effort into this until I know we are stuck going this direction.
Currently, the expand() function has a relatively expensive checked
against the 2MiB aligned pfn's struct page.  I do not know of a way to
eliminate that check against the other page as the first reference we
see for a page is in the middle of that 2MiB aligned range.

To identify this as an area of concern, we had booted with a simulator,
setting watch points on the struct page array region once the
Uninitialized flag was set and maintaining that until it was cleared.

Thanks,
Robin
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