Re: [PATCH V4 1/3] mm/sparsemem: Enable vmem_altmap support in vmemmap_populate_basepages()

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 07/06/2020 02:33 PM, David Hildenbrand wrote:
>>  	return 0;
>> @@ -1505,7 +1505,7 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>>  	int err;
>>  
>>  	if (end - start < PAGES_PER_SECTION * sizeof(struct page))
>> -		err = vmemmap_populate_basepages(start, end, node);
>> +		err = vmemmap_populate_basepages(start, end, node, NULL);
>>  	else if (boot_cpu_has(X86_FEATURE_PSE))
>>  		err = vmemmap_populate_hugepages(start, end, node, altmap);
>>  	else if (altmap) {
> 
> It's somewhat weird that we don't allocate basepages from altmap on x86
> (both for sub-sections and without PSE). I wonder if we can simply
> unlock that with your change. Especially, also handle the
> !X86_FEATURE_PSE case below properly with an altmap.
> 
> a) all hw with PMEM has PSE - except special QEMU setups, so nobody
> cared to implement. For the sub-section special case, nobody cared about
> a handfull of memmap not ending up on the altmap. (but it's still wasted
> system memory IIRC).
> 
> b) the pagetable overhead for small pages is not-neglectable and might
> result in similar issues as solved by the switch to altmap on very huge
> PMEM (with small amount of system RAM).
> 
> I guess it is due to a).

Hmm, I assume these are some decisions that x86 platform will have to
make going forward in a subsequent patch as the third patch does for
the arm64 platform. But it is clearly beyond the scope of this patch
which never intended to change existing behavior on a given platform.

> 
> [...]
> 
>>  
>> -pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, int node)
>> +pte_t * __meminit vmemmap_pte_populate(pmd_t *pmd, unsigned long addr, int node,
>> +				       struct vmem_altmap *altmap)
>>  {
>>  	pte_t *pte = pte_offset_kernel(pmd, addr);
>>  	if (pte_none(*pte)) {
>>  		pte_t entry;
>> -		void *p = vmemmap_alloc_block_buf(PAGE_SIZE, node);
>> +		void *p;
>> +
>> +		if (altmap)
>> +			p = altmap_alloc_block_buf(PAGE_SIZE, altmap);
>> +		else
>> +			p = vmemmap_alloc_block_buf(PAGE_SIZE, node);
>>  		if (!p)
>>  			return NULL;
> 
> I was wondering if
> 
> if (altmap)
> 	p = altmap_alloc_block_buf(PAGE_SIZE, altmap);
> if (!p)
> 	p = vmemmap_alloc_block_buf(PAGE_SIZE, node);
> if (!p)
> 	return NULL
> 
> Would make sense. But I guess this isn't really relevant in practice,
> because the altmap is usually sized properly.
> 
> In general, LGTM.

Okay, I assume that no further changes are required here.