LKML Archive on lore.kernel.org
 help / color / Atom feed
From: David Hildenbrand <david@redhat.com>
To: Mike Rapoport <rppt@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
	Baoquan He <bhe@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	Mel Gorman <mgorman@suse.de>, Michal Hocko <mhocko@kernel.org>,
	Mike Rapoport <rppt@linux.ibm.com>, Qian Cai <cai@lca.pw>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vlastimil Babka <vbabka@suse.cz>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org
Subject: Re: [PATCH v3 1/2] x86/setup: don't remove E820_TYPE_RAM for pfn 0
Date: Wed, 13 Jan 2021 13:56:45 +0100
Message-ID: <6ba6bde3-1520-5cd0-f987-32d543f0b79f@redhat.com> (raw)
In-Reply-To: <20210111194017.22696-2-rppt@kernel.org>

On 11.01.21 20:40, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
> 
> The first 4Kb of memory is a BIOS owned area and to avoid its allocation
> for the kernel it was not listed in e820 tables as memory. As the result,
> pfn 0 was never recognised by the generic memory management and it is not a
> part of neither node 0 nor ZONE_DMA.
> 
> If set_pfnblock_flags_mask() would be ever called for the pageblock
> corresponding to the first 2Mbytes of memory, having pfn 0 outside of
> ZONE_DMA would trigger
> 
> 	VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
> 
> Along with reserving the first 4Kb in e820 tables, several first pages are
> reserved with memblock in several places during setup_arch(). These
> reservations are enough to ensure the kernel does not touch the BIOS area
> and it is not necessary to remove E820_TYPE_RAM for pfn 0.
> 
> Remove the update of e820 table that changes the type of pfn 0 and move the
> comment describing why it was done to trim_low_memory_range() that reserves
> the beginning of the memory.
> 
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
>  arch/x86/kernel/setup.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 740f3bdb3f61..3412c4595efd 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -660,17 +660,6 @@ 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.
> @@ -728,6 +717,15 @@ 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));
>  }
>  	
> 

The only somewhat-confusing thing is that in-between
e820__memblock_setup() and trim_low_memory_range(), we already have
memblock allocations. So [0..4095] might look like ordinary memory until
we reserve it later on.

E.g., reserve_real_mode() does a

mem = memblock_find_in_range(0, 1<<20, size, PAGE_SIZE);
...
memblock_reserve(mem, size);
set_real_mode_mem(mem);

which looks kind of suspicious to me. Most probably I am missing
something, just wanted to point that out. We might want to do such
trimming/adjustments before any kind of allocations.

-- 
Thanks,

David / dhildenb


  parent reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-11 19:40 [PATCH v3 0/2] mm: fix initialization of struct page for holes in memory layout Mike Rapoport
2021-01-11 19:40 ` [PATCH v3 1/2] x86/setup: don't remove E820_TYPE_RAM for pfn 0 Mike Rapoport
2021-01-13  8:56   ` Oscar Salvador
2021-01-13 11:23     ` Mike Rapoport
2021-01-13 12:56   ` David Hildenbrand [this message]
2021-01-13 15:35     ` Mike Rapoport
2021-01-21 13:25   ` Borislav Petkov
2021-01-11 19:40 ` [PATCH v3 2/2] mm: fix initialization of struct page for holes in memory layout Mike Rapoport
2021-02-01  9:14   ` David Hildenbrand
2021-02-01  9:39     ` Baoquan He
2021-02-01 14:12       ` Mike Rapoport
2021-01-12  0:58 ` [PATCH v3 0/2] " Andrew Morton
2021-01-12  5:25   ` Mike Rapoport

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=6ba6bde3-1520-5cd0-f987-32d543f0b79f@redhat.com \
    --to=david@redhat.com \
    --cc=aarcange@redhat.com \
    --cc=akpm@linux-foundation.org \
    --cc=bhe@redhat.com \
    --cc=bp@alien8.de \
    --cc=cai@lca.pw \
    --cc=hpa@zytor.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mgorman@suse.de \
    --cc=mhocko@kernel.org \
    --cc=mingo@redhat.com \
    --cc=rppt@kernel.org \
    --cc=rppt@linux.ibm.com \
    --cc=tglx@linutronix.de \
    --cc=vbabka@suse.cz \
    --cc=x86@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lore.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lore.kernel.org/lkml/9 lkml/git/9.git
	git clone --mirror https://lore.kernel.org/lkml/10 lkml/git/10.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git