linux-kernel - Re: [PATCH V3 1/3] mm/memory_hotplug: Prevalidate the address range being added with platform

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Message-ID: <9916f217-ec29-33ff-a260-7a26792d23a1@redhat.com>
Date:   Fri, 22 Jan 2021 10:18:21 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, hca@...ux.ibm.com,
        catalin.marinas@....com
Cc:     Oscar Salvador <osalvador@...e.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 1/3] mm/memory_hotplug: Prevalidate the address range
 being added with platform


> +/*
> + * Platforms should define arch_get_mappable_range() that provides
> + * maximum possible addressable physical memory range for which the
> + * linear mapping could be created. The platform returned address
> + * range must adhere to these following semantics.
> + *
> + * - range.start <= range.end
> + * - Range includes both end points [range.start..range.end]
> + *
> + * There is also a fallback definition provided here, allowing the
> + * entire possible physical address range in case any platform does
> + * not define arch_get_mappable_range().
> + */
> +struct range __weak arch_get_mappable_range(void)
> +{
> +	struct range memhp_range = {
> +		.start = 0UL,
> +		.end = -1ULL,
> +	};
> +	return memhp_range;
> +}
> +
> +struct range memhp_get_pluggable_range(bool need_mapping)
> +{
> +	const u64 max_phys = (1ULL << (MAX_PHYSMEM_BITS + 1)) - 1;

Sorry, thought about that line a bit more, and I think this is just
wrong (took me longer to realize as it should). The old code used this
calculation to print the limit only (in a wrong way), let's recap:

Assume MAX_PHYSMEM_BITS=32

	max_phys = (1ULL << (32 + 1)) - 1 = 0x1ffffffffull;

Ehm, these are 33 bit.

OTOH, old code checked for

	if (max_addr >> MAX_PHYSMEM_BITS) {

Which makes sense, because

	0x1ffffffffull >> 32 = 1

results in "true", meaning it's to big, while

	0xffffffffull >> 32 = 0

correctly results in "false", meaning the address is fine.



So, this should just be

const u64 max_phys = 1ULL << MAX_PHYSMEM_BITS;

(similarly as calculated in virito-mem code, or in kernel/resource.c)


> +	struct range memhp_range;
> +
> +	if (need_mapping) {
> +		memhp_range = arch_get_mappable_range();
> +		if (memhp_range.start > max_phys) {
> +			memhp_range.start = 0;
> +			memhp_range.end = 0;
> +		}
> +		memhp_range.end = min_t(u64, memhp_range.end, max_phys);
> +	} else {
> +		memhp_range.start = 0;
> +		memhp_range.end = max_phys;
> +	}
> +	return memhp_range;
> +}
> +EXPORT_SYMBOL_GPL(memhp_get_pluggable_range);


-- 
Thanks,

David / dhildenb