Messages in this thread | | | Date | Sun, 8 Apr 2018 16:20:38 +0800 | From | Baoquan He <> | Subject | Re: [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 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.
#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
| |