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

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Message-ID: <897c31ba-d3bd-b694-8c87-82e784a60c51@arm.com>
Date:   Fri, 22 Jan 2021 16:11:25 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     David Hildenbrand <david@...hat.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


On 1/22/21 2:48 PM, David Hildenbrand wrote:
> 
>> +/*
>> + * 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)

Should this be 1ULL << MAX_PHYSMEM_BITS - 1 instead ? Currently there are
three usage for this variable in the function.

- if (mhp_range.start > max_phys)
- mhp_range.end = min_t(u64, mhp_range.end, max_phys)
- mhp_range.end = max_phys

mhp_range.end being always inclusive on the higher end and could be maximum
(1ULL << MAX_PHYSMEM_BITS - 1) which is 0xFFFFFFFF instead of 0x100000000
when (1ULL << MAX_PHYSMEM_BITS) is followed for a 32 bit system. This seems
consistent with the default fallback (range.end = -1ULL) defined above.

> 
> 
>> +	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);
> 
>