Subject: Re: [PATCH v6 4/7] mm/memory_hotplug: Support memmap_on_memory when memmap is not aligned to pageblocks

From: Aneesh Kumar K V <[email-masked]>
Date: Thu, 27 Jul 2023 14:57:17 +0530

On 7/27/23 2:53 PM, Michal Hocko wrote:
> On Thu 27-07-23 13:32:29, Aneesh Kumar K.V wrote:
> [...]
>> +	if (mode == MEMMAP_ON_MEMORY_FORCE) {
>> +		unsigned long memmap_pages = memory_block_memmap_on_memory_pages();
>> +
>> +		pr_info_once("Memory hotplug will reserve %ld pages in each memory block\n",
>> +			     memmap_pages - PFN_UP(memory_block_memmap_size()));
>> +	}
>> +	return 0;
>> +}
> 
> Why should we print this only for the forced case? Isn't that
> interesting for any on memory memmap? Also is this the above sufficient
> on its own? the size depends on the block size and that can vary.
> I think it would make more sense to print the block size and the vmemmap
> reservation and for the force case also any wasted amount on top (if
> any).
> 

For the other cases the space is completely used by for struct page allocation. What
the information is indicating here is that for each memblock we add we are loosing/wasting so many pages. 
May be I should have used the term "waste" instead of "reserve" ?

-aneesh