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

[Johannes Weiner <hannes@cmpxchg.org>]

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.

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