Messages in this thread Patch in this message | | | Subject | Re: [PATCH v3 4/4] mm/sparse: Optimize memmap allocation during sparse_init() | From | Dave Hansen <> | Date | Fri, 6 Apr 2018 07:50:18 -0700 |
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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).
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.
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.
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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.
> @@ -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?
> > 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.
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