[PATCH] powerpc/mm: Ensure Huge-page memory is free before allocation

[Michael Ellerman]

Vaibhav Jain <vaibhav at linux.ibm.com> writes:
> We recently discovered an bug where physical memory meant for
> allocation of Huge-pages was inadvertently allocated by another component
> during early boot.

Can you give me some more detail on what that was? You're seemingly the
only person who's ever hit this :)

> The behavior of memblock_reserve() where it wont
> indicate whether an existing reserved block overlaps with the
> requested reservation only makes such bugs hard to investigate.
>
> Hence this patch proposes adding a memblock reservation check in
> htab_dt_scan_hugepage_blocks() just before call to memblock_reserve()
> to ensure that the physical memory thats being reserved for is not
> already reserved by someone else. In case this happens we panic the
> the kernel to ensure that user of this huge-page doesn't accidentally
> stomp on memory allocated to someone else.

Do we really need to panic? Can't we just leave the block alone and not
register it as huge page memory? With a big warning obviously.

cheers

> diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
> index 28ced26f2a00..a05be3adb8c9 100644
> --- a/arch/powerpc/mm/book3s64/hash_utils.c
> +++ b/arch/powerpc/mm/book3s64/hash_utils.c
> @@ -516,6 +516,11 @@ static int __init htab_dt_scan_hugepage_blocks(unsigned long node,
>  	printk(KERN_INFO "Huge page(16GB) memory: "
>  			"addr = 0x%lX size = 0x%lX pages = %d\n",
>  			phys_addr, block_size, expected_pages);
> +
> +	/* Ensure no one else has reserved memory for huge pages before */
> +	BUG_ON(memblock_is_region_reserved(phys_addr,
> +					   block_size * expected_pages));
> +
>  	if (phys_addr + block_size * expected_pages <= memblock_end_of_DRAM()) {
>  		memblock_reserve(phys_addr, block_size * expected_pages);
>  		pseries_add_gpage(phys_addr, block_size, expected_pages);
> -- 
> 2.21.0