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

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

> On Sun, May 4, 2008 at 8:34 AM, Johannes Weiner <hannes@saeurebad.de> wrote:
>>
>> Hi,
>>
>> Johannes Weiner <hannes@saeurebad.de> writes:
>>
>> > 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.
>>
>> In the long term, this would have to be implemented no matter if
>> rootmem/bootmem2 gets merged or not, because bootmem suffers the same
>> problem, right?
>>
>>
>> >> 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.
>>
>> This is non-helping crap, please excuse me.
>>
>>
>> > 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.
>>
>> Hm, we have two choices: Either we introduce a new API that requests the
>> arch code to register not only node ranges but also subranges on that
>> node, or we won't garuantee that you get all memory on the node you
>> specified. Correct?
>>
>> The first option would be what you have proposed, I think.
>
> 1. current bootmem, add not_used_map to bdata.
> 2. or in bootmem2, use pages_offset struct for every range... so one
> node could have several ranges.

I think I found a solution, please have a look at the bootmem2 patches
(coming soon).

Hannes


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