linux-kernel - Re: [RFC/RFT PATCH 1/3] memblock: update initialization of reserved pages

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Message-ID: <0c48f98c-7454-1458-15a5-cc5a7e1fb7cd@redhat.com>
Date:   Wed, 14 Apr 2021 17:12:11 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC/RFT PATCH 1/3] memblock: update initialization of reserved
 pages

On 07.04.21 19:26, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@...ux.ibm.com>
> 
> The struct pages representing a reserved memory region are initialized
> using reserve_bootmem_range() function. This function is called for each
> reserved region just before the memory is freed from memblock to the buddy
> page allocator.
> 
> The struct pages for MEMBLOCK_NOMAP regions are kept with the default
> values set by the memory map initialization which makes it necessary to
> have a special treatment for such pages in pfn_valid() and
> pfn_valid_within().

I assume these pages are never given to the buddy, because we don't have 
a direct mapping. So to the kernel, it's essentially just like a memory 
hole with benefits.

I can spot that we want to export such memory like any special memory 
thingy/hole in /proc/iomem -- "reserved", which makes sense.

I would assume that MEMBLOCK_NOMAP is a special type of *reserved* 
memory. IOW, that for_each_reserved_mem_range() should already succeed 
on these as well -- we should mark anything that is MEMBLOCK_NOMAP 
implicitly as reserved. Or are there valid reasons not to do so? What 
can anyone do with that memory?

I assume they are pretty much useless for the kernel, right? Like other 
reserved memory ranges.


> 
> Split out initialization of the reserved pages to a function with a
> meaningful name and treat the MEMBLOCK_NOMAP regions the same way as the
> reserved regions and mark struct pages for the NOMAP regions as
> PageReserved.
> 
> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> ---
>   mm/memblock.c | 23 +++++++++++++++++++++--
>   1 file changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index afaefa8fc6ab..6b7ea9d86310 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -2002,6 +2002,26 @@ static unsigned long __init __free_memory_core(phys_addr_t start,
>   	return end_pfn - start_pfn;
>   }
>   
> +static void __init memmap_init_reserved_pages(void)
> +{
> +	struct memblock_region *region;
> +	phys_addr_t start, end;
> +	u64 i;
> +
> +	/* initialize struct pages for the reserved regions */
> +	for_each_reserved_mem_range(i, &start, &end)
> +		reserve_bootmem_region(start, end);
> +
> +	/* and also treat struct pages for the NOMAP regions as PageReserved */
> +	for_each_mem_region(region) {
> +		if (memblock_is_nomap(region)) {
> +			start = region->base;
> +			end = start + region->size;
> +			reserve_bootmem_region(start, end);
> +		}
> +	}
> +}
> +
>   static unsigned long __init free_low_memory_core_early(void)
>   {
>   	unsigned long count = 0;
> @@ -2010,8 +2030,7 @@ static unsigned long __init free_low_memory_core_early(void)
>   
>   	memblock_clear_hotplug(0, -1);
>   
> -	for_each_reserved_mem_range(i, &start, &end)
> -		reserve_bootmem_region(start, end);
> +	memmap_init_reserved_pages();
>   
>   	/*
>   	 * We need to use NUMA_NO_NODE instead of NODE_DATA(0)->node_id
> 


-- 
Thanks,

David / dhildenb