linux-kernel - Re: [RFC 4/4] Sparse initialization of struct page array.

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Message-ID: <20130715182615.GF3421@sgi.com>
Date:	Mon, 15 Jul 2013 13:26:15 -0500
From:	Robin Holt <holt@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Nathan Zimmer <nzimmer@....com>, Yinghai Lu <yinghai@...nel.org>,
	Robin Holt <holt@....com>, Ingo Molnar <mingo@...nel.org>,
	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>,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [RFC 4/4] Sparse initialization of struct page array.

On Mon, Jul 15, 2013 at 10:54:38AM -0700, H. Peter Anvin wrote:
> On 07/15/2013 10:45 AM, Nathan Zimmer wrote:
> > 
> > I hadn't actually been very happy with having a PG_uninitialized2mib flag.
> > It implies if we want to jump to 1Gb pages we would need a second flag,
> > PG_uninitialized1gb, for that.  I was thinking of changing it to
> > PG_uninitialized and setting page->private to the correct order.
> > Thoughts?
> > 
> 
> Seems straightforward.  The bigger issue is the amount of overhead we
> cause by having to check upstack for the initialization status of the
> superpages.
> 
> I'm concerned, obviously, about lingering overhead that is "forever".
> That being said, in the absolutely worst case we could have a counter to
> the number of uninitialized pages which when it hits zero we do a static
> switch and switch out the initialization code (would have to be undone
> on memory hotplug, of course.)

Is there a fairly cheap way to determine definitively that the struct
page is not initialized?

I think this patch set can change fairly drastically if we have that.
I think I will start working up those changes and code a heavy-handed
check until I hear of an alternative way to cheaply check.

Thanks,
Robin
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