Re: [PATCH] mm: bootmem: Check pfn_valid() before accessing struct page

[Dave Hansen]

On 05/27/2014 07:10 AM, Matt Fleming wrote:
> We need to check that a pfn is valid before handing it to pfn_to_page()
> since on low memory systems with CONFIG_HIGHMEM=n it's possible that a
> pfn may not have a corresponding struct page.
> 
> This is in fact the case for one of Alan's machines where some of the
> EFI boot services pages live in highmem, and running a kernel without
> CONFIG_HIGHMEM enabled results in the following oops
...
> diff --git a/mm/bootmem.c b/mm/bootmem.c
> index 90bd3507b413..406e9cb1d58c 100644
> --- a/mm/bootmem.c
> +++ b/mm/bootmem.c
> @@ -164,6 +164,9 @@ void __init free_bootmem_late(unsigned long physaddr, unsigned long size)
>  	end = PFN_DOWN(physaddr + size);
>  
>  	for (; cursor < end; cursor++) {
> +		if (!pfn_valid(cursor))
> +			continue;
> +
>  		__free_pages_bootmem(pfn_to_page(cursor), 0);
>  		totalram_pages++;
>  	}

I don't think this is quite right.  pfn_valid() tells us whether we have
a 'struct page' there or not.  *BUT*, it does not tell us whether it is
RAM that we can actually address and than can be freed in to the buddy
allocator.

I think sparsemem is where this matters.  Let's say mem= caused lowmem
to end in the middle of a section (or that 896MB wasn't
section-aligned).  Then someone calls free_bootmem_late() on an area
that is in the last section, but _above_ max_mapnr.  It'll be
pfn_valid(), we'll free it in to the buddy allocator, and we'll blam the
first time we try to write to a bogus vaddr after a phys_to_virt().

At a higher level, I don't like the idea of the bootmem code papering
over bugs when somebody calls in to it trying to _free_ stuff that's not
memory (as far as the kernel is concerned).

I think the right thing to do is to call in to the e820 code and see if
the range is E820_RAM before trying to bootmem-free it.
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