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

Johannes Weiner <hannes@saeurebad.de> writes:

> 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.

One thing I have found is that __rootmem_alloc_node can not garuantee
that the memory it returns is on the requested node right now.

I will include the fix in the next version.

Hannes


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