linux-kernel - Re: [PATCH v2 0/3] support for broken memory modules (BadRAM)

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Message-ID: <4E0230CD.1030505@zytor.com>
Date:	Wed, 22 Jun 2011 11:13:33 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Stefan Assmann <sassmann@...nic.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, tony.luck@...el.com,
	andi@...stfloor.org, mingo@...e.hu, rick@...rein.org,
	rdunlap@...otime.net, Nancy Yuen <yuenn@...gle.com>,
	Michael Ditto <mditto@...gle.com>
Subject: Re: [PATCH v2 0/3] support for broken memory modules (BadRAM)

On 06/22/2011 11:00 AM, Andrew Morton wrote:
> : 
> : Second, the BadRAM patch expands the address patterns from the command
> : line into individual entries in the kernel's e820 table.  The e820
> : table is a fixed buffer that supports a very small, hard coded number
> : of entries (128).  We require a much larger number of entries (on
> : the order of a few thousand), so much of the google kernel patch deals
> : with expanding the e820 table.

This has not been true for a long time.

> I have a couple of thoughts here:
> 
> - If this patchset is merged and a major user such as google is
>   unable to use it and has to continue to carry a separate patch then
>   that's a regrettable situation for the upstream kernel.
> 
> - Google's is, afaik, the largest use case we know of: zillions of
>   machines for a number of years.  And this real-world experience tells
>   us that the badram patchset has shortcomings.  Shortcomings which we
>   can expect other users to experience.
> 
> So.  What are your thoughts on these issues?

I think a binary structure fed as a linked list data object makes a lot
more sense.  We already support feeding e820 entries in this way,
bypassing the 128-entry limitation of the fixed table in the zeropage.

The main issue then is priority; in particular memory marked UNUSABLE
(type 5) in the fed-in e820 map will of course overlap entries with
normal RAM (type 1) information in the native map; we need to make sure
that the type 5 information takes priority.

	-hpa
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