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

[Baoquan He]

On 04/08/18 at 04:20pm, Baoquan He wrote:
> 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
> avoided.
> 
> > 
> > ---
> > 
> > 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.

Oops, I was wrong. Here nr_present_sections is the number of present
sections, while section_nr is index of all sections. If decide to do,
can add check like below?

	if (idx_present >= nr_present_sections) {
		pr_err("idx_present goes beyond nr_present_sections, xxxx \n");
		break;
	}

> 
> #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.
> 
> Thanks
> Baoquan