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Date:   Thu, 4 Feb 2021 20:19:25 +0200
From:   Mike Rapoport <rppt@linux.ibm.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Chris Wilson <chris@chris-wilson.co.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        stable@vger.kernel.org
Subject: Re: Linux 5.11-rc5
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On Mon, Jan 25, 2021 at 12:49:39PM -0800, Linus Torvalds wrote:
> On Mon, Jan 25, 2021 at 12:35 PM Chris Wilson <chris@chris-wilson.co.uk> wrote:
> 
> Mike: should we perhaps revert the first patch too (commit
> bde9cfa3afe4: "x86/setup: don't remove E820_TYPE_RAM for pfn 0")?

Unfortunately, I was too optimistic and didn't take into account that this
commit changes the way /dev/mem sees the first page of memory.

There were reports of slackware users about issues with lilo after upgrade
from 5.10.11 to 5.10.12

https://www.linuxquestions.org/questions/slackware-14/slackware-current-lilo-vesa-warnings-after-recent-updates-4175689617/#post6214439 

The root cause is that lilo is no longer able to access the first memory
page via /dev/mem because its type was changed from E820_TYPE_RESERVED to
E820_TYPE_RAM, so this became a part of the "System RAM" resource and
devmem_is_allowed() considers it disallowed area.

So here's the revert of bde9cfa3afe4 as well.

From a7fdc4117010d393dd77b99da5b573a5c98453ce Mon Sep 17 00:00:00 2001
From: Mike Rapoport <rppt@linux.ibm.com>
Date: Thu, 4 Feb 2021 20:12:37 +0200
Subject: [PATCH] Revert "x86/setup: don't remove E820_TYPE_RAM for pfn 0"

This reverts commit bde9cfa3afe4324ec251e4af80ebf9b7afaf7afe.

Changing the first memory page type from E820_TYPE_RESERVED to
E820_TYPE_RAM makes it a part of "System RAM" resource rather than a
reserved resource and this in turn causes devmem_is_allowed() to treat is
as area that can be accessed but it is filled with zeroes instead of the
actual data as previously.

The change in /dev/mem output causes lilo to fail as was reported at
slakware users forum [1], and probably other legacy applications will
experience similar problems.

[1] https://www.linuxquestions.org/questions/slackware-14/slackware-current-lilo-vesa-warnings-after-recent-updates-4175689617/#post6214439

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
---
 arch/x86/kernel/setup.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 3412c4595efd..740f3bdb3f61 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -660,6 +660,17 @@ static void __init trim_platform_memory_ranges(void)
 
 static void __init trim_bios_range(void)
 {
+	/*
+	 * A special case is the first 4Kb of memory;
+	 * This is a BIOS owned area, not kernel ram, but generally
+	 * not listed as such in the E820 table.
+	 *
+	 * This typically reserves additional memory (64KiB by default)
+	 * since some BIOSes are known to corrupt low memory.  See the
+	 * Kconfig help text for X86_RESERVE_LOW.
+	 */
+	e820__range_update(0, PAGE_SIZE, E820_TYPE_RAM, E820_TYPE_RESERVED);
+
 	/*
 	 * special case: Some BIOSes report the PC BIOS
 	 * area (640Kb -> 1Mb) as RAM even though it is not.
@@ -717,15 +728,6 @@ early_param("reservelow", parse_reservelow);
 
 static void __init trim_low_memory_range(void)
 {
-	/*
-	 * A special case is the first 4Kb of memory;
-	 * This is a BIOS owned area, not kernel ram, but generally
-	 * not listed as such in the E820 table.
-	 *
-	 * This typically reserves additional memory (64KiB by default)
-	 * since some BIOSes are known to corrupt low memory.  See the
-	 * Kconfig help text for X86_RESERVE_LOW.
-	 */
 	memblock_reserve(0, ALIGN(reserve_low, PAGE_SIZE));
 }
 	
-- 
2.29.2


>                 Linus

-- 
Sincerely yours,
Mike.