Re: [PATCH 2/2] mm/sparse.c: Add nr_present_sections to change the mem_map allocation

[Baoquan He]

On 02/01/18 at 05:49am, Dave Hansen wrote:
> On 02/01/2018 02:16 AM, Kirill A. Shutemov wrote:
> > On Thu, Feb 01, 2018 at 03:19:56PM +0800, Baoquan He wrote:
> >> In sparse_init(), we allocate usemap_map and map_map which are pointer
> >> array with the size of NR_MEM_SECTIONS. The memory consumption can be
> >> ignorable in 4-level paging mode. While in 5-level paging, this costs
> >> much memory, 512M. Kdump kernel even can't boot up with a normal
> >> 'crashkernel=' setting.
> >>
> >> Here add a new variable to record the number of present sections. Let's
> >> allocate the usemap_map and map_map with the size of nr_present_sections.
> >> We only need to make sure that for the ith present section, usemap_map[i]
> >> and map_map[i] store its usemap and mem_map separately.
> >>
> >> This change can save much memory on most of systems. Anytime, we should
> >> avoid to define array or allocate memory with the size of NR_MEM_SECTIONS.
> > That's very desirable outcome. But I don't know much about sparsemem.
> 
> ... with the downside being that we can no longer hot-add memory that
> was not part of the original, present sections.
> 
> Is that OK?

Thanks for looking into this, Dave!

I suppose these functions changed here are only called during system
bootup, namely in paging_init(). Hot-add memory goes in a different
path, __add_section() -> sparse_add_one_section(), different called
functions.