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

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Message-ID: <5e133a5e-41bb-9d6b-f76e-a96d3efe0f5e@redhat.com>
Date:   Fri, 22 Jan 2021 11:42:17 +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

On 22.01.21 11:41, Anshuman Khandual wrote:
> 
> 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

Yes, obviously, sorry, forgot the -1.

-- 
Thanks,

David / dhildenb