linux-kernel - [PATCH] x86/mm: Limit 2/4M size calculation to x86

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Date:	Fri, 31 Aug 2012 09:31:39 -0700
From:	Stefan Bader <stefan.bader@...onical.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:	Avi Kivity <avi@...hat.com>, Cong Wang <xiyou.wangcong@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Yinghai Lu <yinghai@...nel.org>,
	Tejun Heo <tj@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Konrad Rezeszutek Wilk <konrad.wilk@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] x86/mm: Limit 2/4M size calculation to x86_32

Avi wrote:
>The fact that the check is only done on i386 and not on x86_64
> may come from one of
>
> - an oversight
> - by the time x86_64 processors came along, the problem with
>   conflicting sizes was resolved
> - the whole thing is bogus
>
> Copying hpa who may be in a position to find out which.

Talking to hpa it is more of the last. For more than just this
reason. Since the whole area of initial page tables seems to be
rather sensitive and easy to break there have been discussions
and plans to come up with a rewrite to improve on all those
shortcomings.

The detail I am not agreeing with hpa is the fixup for the
immediate breakage at head. IMO right now the code just has
regressed and that should be fixed as soon as possible.
Plus doing a specific and small fix allows that to be applicable
to stable (which again still depends on things being upstream).

Hence the re-send in the hope that on the larger scale the may
be agreement on the immediate fix. I am not doubting the usefulness
or need of a better solution, but I think that having a remedy of
the current situation just until then has enough benefit to be
considered.

-Stefan

>From 1d5cc3971716a039c91abc18cb6f9bcbe5dde490 Mon Sep 17 00:00:00 2001
From: Stefan Bader <stefan.bader@...onical.com>
Date: Fri, 13 Jul 2012 15:16:33 +0200
Subject: [PATCH] x86/mm: Limit 2/4M size calculation to x86_32

commit 722bc6b (x86/mm: Fix the size calculation of mapping tables)
did modify the extra space calculation for mapping tables in order
to make up for the first 2/4M memory range using 4K pages.
However this setup is only used when compiling for 32bit. On 64bit
there is only the trailing area of 4K pages (which is already added).

The code was already adapted once for things went wrong on a 8TB
machine (bd2753b x86/mm: Only add extra pages count for the first memory
range during pre-allocation early page table space), but it looks a bit
like it currently would overdo things for 64bit.
I only noticed while bisecting for the reason I could not make a crash
kernel boot (which ended up on this patch).

Signed-off-by: Stefan Bader <stefan.bader@...onical.com>
Cc: stable@...r.kernel.org # v3.5
Cc: WANG Cong <xiyou.wangcong@...il.com>
Cc: Yinghai Lu <yinghai@...nel.org>
Cc: Tejun Heo <tj@...nel.org>
---
 arch/x86/mm/init.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index e0e6990..28a1c99 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -60,10 +60,11 @@ static void __init find_early_table_space(struct map_range *mr, unsigned long en
 		extra = end - ((end>>PMD_SHIFT) << PMD_SHIFT);
 #ifdef CONFIG_X86_32
 		extra += PMD_SIZE;
-#endif
+
 		/* The first 2/4M doesn't use large pages. */
 		if (mr->start < PMD_SIZE)
 			extra += mr->end - mr->start;
+#endif

 		ptes = (extra + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	} else
-- 
1.7.10.4

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