Re: [PATCH v3 4/4] mm/sparse: Optimize memmap allocation during sparse_init()

[Dave Hansen]

On 04/14/2018 07:19 PM, Baoquan He wrote:
>>> Yes, this place is the hardest to understand. The temorary arrays are
>>> allocated beforehand with the size of 'nr_present_sections'. The error
>>> paths you mentioned is caused by allocation failure of mem_map or
>>> map_map, but whatever it's error or success paths, the sections must be
>>> marked as present in memory_present(). Error or success paths happened
>>> in alloc_usemap_and_memmap(), while checking if it's erorr or success
>>> paths happened in the last for_each_present_section_nr() of
>>> sparse_init(), and clear the ms->section_mem_map if it goes along error
>>> paths. This is the key point of this new allocation way.
>> I think you owe some commenting because this is so hard to understand.
> I can arrange and write a code comment above sparse_init() according to
> this patch's git log, do you think it's OK?
> 
> Honestly, it took me several days to write code, while I spent more
> than one week to write the patch log. Writing patch log is really a
> headache to me.

I often find the same: writing the code is the easy part.  Explaining
why it is right is the hard part.