linux-kernel - Re: [RFC 0/2] Rootmem: boot-time memory allocator

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Date:	Sun, 04 May 2008 16:17:11 +0200
From:	Johannes Weiner <hannes@...urebad.de>
To:	"Yinghai Lu" <yhlu.kernel@...il.com>
Cc:	"Ingo Molnar" <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, "Andi Kleen" <andi@...stfloor.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>
Subject: Re: [RFC 0/2] Rootmem: boot-time memory allocator

Hi Yinghai,

Johannes Weiner <hannes@...urebad.de> writes:

> Hi,
>
> "Yinghai Lu" <yhlu.kernel@...il.com> writes:
>
>> On Sat, May 3, 2008 at 10:54 AM, Ingo Molnar <mingo@...e.hu> wrote:
>>>
>>>  * Johannes Weiner <hannes@...urebad.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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