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SubjectRe: [RFC 0/2] Rootmem: boot-time memory allocator
Date
Hi,

"Yinghai Lu" <yhlu.kernel@gmail.com> writes:

> On Sat, May 3, 2008 at 10:54 AM, Ingo Molnar <mingo@elte.hu> wrote:
>>
>> * Johannes Weiner <hannes@saeurebad.de> wrote:
>>
>> > I was spending some time and work on the bootmem allocator the last
>> > few weeks and came to the conclusion that its current design is not
>> > appropriate anymore.
>> >
>> > As Ingo said in another email, NUMA technologies will become weirder,
>> > nodes whose PFNs span other nodes for example and it makes bootmem
>> > code become an unreadable mess.
>> >
>> > So I sat down two days ago and rewrote the allocator, here is the
>> > result: rootmem!
>>
>> hehe :-)
>>
>>
>> > The biggest difference to the old design is that there is only one
>> > bitmap for all PFNs of all nodes together, so the overlapping PFN
>> > problems simply dissolve and fun like allocations crossing node
>> > boundaries work implicitely. The new API requires every node used by
>> > the allocator to be registered and after that the bitmap gets
>> > allocated and the allocator enabled.
>> >
>> > I chose to add a new allocator rather than replacing bootmem at once
>> > because that would have required all callsites to switch in one go,
>> > which would be a lot. The new allocator can be adopted more slowly
>> > and I added a compatibility API for everything besides actually
>> > setting up the allocator. When the last user dies, bootmem can be
>> > dropped completely (including pgdat->bdata, whee..)
>> >
>> > The main ideas from bootmem have been stolen^W preserved but the new
>> > design allowed me to shrink the code a lot and express things more
>> > simple and clear:
>> >
>> > $ sloc.awk < mm/bootmem.c
>> > 455 lines of code, 65 lines of comments (520 lines total)
>> >
>> > $ sloc.awk < mm/rootmem.c
>> > 243 lines of code, 96 lines of comments (339 lines total)
>>
>> amazing!
>>
>> i'd still suggest to keep it all named bootmem though :-/ How about
>> bootmem2.c and then renaming it back to bootmem.c, once the last user is
>> gone? That would save people from having to rename whole chapters in
>> entire books ;-)
>
> for spanning support node0:0-2g, 4-6g; node1: 2-4g, 6-8g, could have
> some problem.

Could you eleborate on that?

> +/*
> + * rootmem_register_node - register a node to rootmem
> + * @nid: node id
> + * @start: first pfn on the node
> + * @end: first pfn after the node
> + *
> + * This function must not be called anymore if the allocator
> + * is already up and running (rootmem_setup() has been called).
> + */
> +void __init rootmem_register_node(int nid, unsigned long start,
> + unsigned long end)
> +{
> + BUG_ON(rootmem_functional);
> +
> + if (start < rootmem_min_pfn)
> + rootmem_min_pfn = start;
> + if (end > rootmem_max_pfn)
> + rootmem_max_pfn = end;
> +
> + rootmem_node_pages[nid] = end - start;
> + rootmem_node_offsets[nid] = start;
> + rootmem_nr_nodes++;
> +}
>
> could change rootmem_node_pages/offsets to be struct array with
> offset, pages, and nid. and every node could several struct. and whole
> array should be sorted with nid.

The whole point is to be agnostic about weird NUMA configs. Right now,
I am pretty proud of the simple data structures and I would avoid
blowing them up again unless there is a hard reason to do so.

Hannes


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