linux-kernel - Re: [PATCH v3 1/2] x86/setup: don't remove E820_TYPE

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Date:   Thu, 21 Jan 2021 14:25:42 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Baoquan He <bhe@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>, Qian Cai <cai@....pw>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org
Subject: Re: [PATCH v3 1/2] x86/setup: don't remove E820_TYPE_RAM for pfn 0

On Mon, Jan 11, 2021 at 09:40:16PM +0200, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@...ux.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@...ux.ibm.com>
> ---
>  arch/x86/kernel/setup.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)

FWIW,

Acked-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette