Re: [PATCH 1/2] memcg: consistently use vmalloc for page_cgroup allocations

Kamezawa Hiroyuki
2013-04-09T02:41:59Z

(2013/04/06 1:25), Johannes Weiner wrote:
> On Fri, Apr 05, 2013 at 04:27:56PM +0400, Glauber Costa wrote:
>> On 04/05/2013 04:06 PM, Johannes Weiner wrote:
>>> On Fri, Apr 05, 2013 at 02:01:11PM +0400, Glauber Costa wrote:
>>>> Right now, allocation for page_cgroup is a bit complicated, dependent on
>>>> a variety of system conditions:
>>>>
>>>> For flat memory, we are likely to need quite big pages, so the page
>>>> allocator won't cut. We are forced to init flatmem mappings very early,
>>>> because if we run after the page allocator is in place those allocations
>>>> will be denied. Flatmem mappings thus resort to the bootmem allocator.
>>>>
>>>> We can fix this by using vmalloc for flatmem mappings. However, we now
>>>> have the situation in which flatmem mapping allocate using vmalloc, but
>>>> sparsemem may or may not allocate with vmalloc. It will try the
>>>> page_allocator first, and retry vmalloc if it fails.
>>>
>>> Vmalloc space is a precious resource on 32-bit systems and harder on
>>> the TLB than the identity mapping.
>>>
>>> It's a last resort thing for when you need an unusually large chunk of
>>> contiguously addressable memory during runtime, like loading a module,
>>> buffers shared with userspace etc.. But here we know, during boot
>>> time, the exact amount of memory we need for the page_cgroup array.
>>>
>>> Code cleanup is not a good reason to use vmalloc in this case, IMO.
>>>
>> This is indeed a code cleanup, but a code cleanup with a side goal:
>> freeing us from the need to register page_cgroup mandatorily at init
>> time. This is done because page_cgroup_init_flatmem will use the bootmem
>> allocator, to avoid the page allocator limitations.
>>
>> What I can try to do, and would happily do, is to try a normal page
>> allocation and then resort to vmalloc if it is too big.
>>
>> Would that be okay to you ?
>
> With the size of page_cgroup right now (2 words), we need half a page
> per MB of represented memory on 32 bit, so booting on a 4GB 32 bit
> machine needs an order-11 (MAX_ORDER) allocation and thus fall back to
> 8MB of the 128MB vmalloc space. A 16GB machine falls back to 32MB, a
> quarter of the vmalloc space.
>
> Now, I think we all agree that these are not necessarily recommended
> configurations but we should not be breaking them for the hell of it
> either.
>
> How about leaving flatmem as it is and have an on-demand allocation
> model that just works with sparsemem? A 128MB section on 64 bit
> "only" needs order-7 pages, but we satisfy order-9 THP allocations all
> the time during runtime, so this may just work.
>

I agree to Johannes' option.

Thanks,
-Kame