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SubjectRe: [PATCH v3 4/4] mm/sparse: Optimize memmap allocation during sparse_init()
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On 04/08/2018 01:20 AM, Baoquan He wrote:
> On 04/06/18 at 07:50am, Dave Hansen wrote:
>> 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))

I was more concerned about the loops that "consume" the section maps.
It seems like they might run over the end of the array.

>>> @@ -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.

Does the large array index make a bunch of lines wrap or something? If
not, I'd just use the long name.

>>> 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.

I think you owe some commenting because this is so hard to understand.

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