Re: [PATCH] mm: cma: allocate pages from CMA if NR_FREE_PAGES approaches low water mark

Marek Szyprowski
2012-11-20T14:49:43Z

Hello,

On 11/20/2012 1:01 AM, Minchan Kim wrote:
> Hi Marek,
>
> On Mon, Nov 12, 2012 at 09:59:42AM +0100, Marek Szyprowski wrote:
> > It has been observed that system tends to keep a lot of CMA free pages
> > even in very high memory pressure use cases. The CMA fallback for movable
>
> CMA free pages are just fallback for movable pages so if user requires many
> user pages, it ends up consuming cma free pages after out of movable pages.
> What do you mean that system tend to keep free pages even in very
> high memory pressure?
> > pages is used very rarely, only when system is completely pruned from
> > MOVABLE pages, what usually means that the out-of-memory even will be
> > triggered very soon. To avoid such situation and make better use of CMA
>
> Why does OOM is triggered very soon if movable pages are burned out while
> there are many cma pages?
>
> It seems I can't understand your point quitely.
> Please make your problem clear for silly me to understand clearly.

Right now running out of 'plain' movable pages is the only possibility to
get movable pages allocated from CMA. On the other hand running out of
'plain' movable pages is very deadly for the system, as movable pageblocks
are also the main fallbacks for reclaimable and non-movable pages.

Then, once we run out of movable pages and kernel needs non-mobable or
reclaimable page (what happens quite often), it usually triggers OOM to
satisfy the memory needs. Such OOM is very strange, especially on a system
with dozen of megabytes of CMA memory, having most of them free at the OOM
event. By high memory pressure I mean the high memory usage.

This patch introduces a heuristics which let kernel to consume free CMA
pages before it runs out of 'plain' movable pages, what is usually enough to
keep some spare movable pages for emergency cases before the reclaim occurs.

Best regards
--
Marek Szyprowski
Samsung Poland R&D Center