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SubjectRe: [PATCH v3 4/4] mm/sparse: Optimize memmap allocation during sparse_init()
On 04/06/18 at 07:50am, Dave Hansen wrote:
> I'm having a really hard time tying all the pieces back together. Let
> me give it a shot and you can tell me where I go wrong.
> On 02/27/2018 07:26 PM, Baoquan He wrote:
> > In sparse_init(), two temporary pointer arrays, usemap_map and map_map
> > are allocated with the size of NR_MEM_SECTIONS.
> In sparse_init(), two temporary pointer arrays, usemap_map and map_map
> are allocated to hold the maps for every possible memory section
> (NR_MEM_SECTIONS). However, we obviously only need the array sized for
> nr_present_sections (introduced in patch 1).

Yes, correct.

> The reason this is a problem is that, with 5-level paging,
> NR_MEM_SECTIONS (8M->512M) went up dramatically and these temporary
> arrays can eat all of memory, like on kdump kernels.

With 5-level paging enabled, MAX_PHYSMEM_BITS changed from 46 to
52. You can see NR_MEM_SECTIONS becomes 64 times of the old value. So
the two temporary pointer arrays eat more memory, 8M -> 8M*64 = 512M.

# define MAX_PHYSMEM_BITS (pgtable_l5_enabled ? 52 : 46)

> This patch does two things: it makes sure to give usemap_map/mem_map a
> less gluttonous size on small systems, and it changes the map allocation
> and handling to handle the now more compact, less sparse arrays.

Yes, because 99.9% of systems do not have PB level of memory, not even TB.
Any place of memory allocatin with the size of NR_MEM_SECTIONS should be

> ---
> The code looks fine to me. It's a bit of a shame that there's no
> verification to ensure that idx_present never goes beyond the shiny new
> nr_present_sections.

This is a good point. Do you think it's OK to replace (section_nr <
NR_MEM_SECTIONS) with (section_nr < nr_present_sections) in below
for_each macro? This for_each_present_section_nr() is only used
during sparse_init() execution.

#define for_each_present_section_nr(start, section_nr) \
for (section_nr = next_present_section_nr(start-1); \
((section_nr >= 0) && \
(section_nr < NR_MEM_SECTIONS) && \
(section_nr <= __highest_present_section_nr)); \
section_nr = next_present_section_nr(section_nr))

> > @@ -583,6 +592,7 @@ void __init sparse_init(void)
> > unsigned long *usemap;
> > unsigned long **usemap_map;
> > int size;
> > + int idx_present = 0;
> I wonder whether idx_present is a good name. Isn't it the number of
> consumed mem_map[]s or usemaps?

Yeah, in sparse_init(), it's the index of present memory sections, and
also the number of consumed mem_map[]s or usemaps. And I remember you
suggested nr_consumed_maps instead. seems nr_consumed_maps is a little
long to index array to make code line longer than 80 chars. How about
name it idx_present in sparse_init(), nr_consumed_maps in
alloc_usemap_and_memmap(), the maps allocation function? I am also fine
to use nr_consumed_maps for all of them.

> >
> > if (!map) {
> > ms->section_mem_map = 0;
> > + idx_present++;
> > continue;
> > }
> >
> This hunk seems logically odd to me. I would expect a non-used section
> to *not* consume an entry from the temporary array. Why does it? The
> error and success paths seem to do the same thing.

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.


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