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 [thread overview]
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
next prev parent reply other threads:[~2021-01-13 12:56 UTC|newest]
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
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