Re: 答复: [PATCH] mm: optimization on page allocation when CMA enabled

[Roman Gushchin]

On Tue, May 02, 2023 at 12:12:28PM +0000, 黄朝阳 (Zhaoyang Huang) wrote:
> > Hi Zhaoyang!
> > 
> > On Fri, Apr 28, 2023 at 07:00:41PM +0800, zhaoyang.huang wrote:
> > > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > >
> > > Please be notice bellowing typical scenario that commit 168676649
> > > introduce, that is, 12MB free cma pages 'help' GFP_MOVABLE to keep
> > > draining/fragmenting U&R page blocks until they shrink to 12MB without
> > > enter slowpath which against current reclaiming policy. This commit change
> > the criteria from hard coded '1/2'
> > > to watermark check which leave U&R free pages stay around WMARK_LOW
> > > when being fallback.
> > 
> > Can you, please, explain the problem you're solving in more details?
> I am trying to solve a OOM problem caused by slab allocation fail as all free pages are MIGRATE_CMA by applying 168676649, which could help to reduce the fault ration from 12/20 to 2/20. I noticed it introduce the phenomenon which I describe above.
> > 
> > If I understand your code correctly, you're effectively reducing the use of cma
> > areas for movable allocations. Why it's good?
> Not exactly. In fact, this commit lead to the use of cma early than it is now, which could help to protect U&R be 'stolen' by GFP_MOVABLE. Imagine this scenario, 30MB total free pages composed of 10MB CMA and 20MB U&R, while zone's watermark low is 25MB. An GFP_MOVABLE allocation can keep stealing U&R pages(don't meet 1/2 criteria) without enter slowpath(zone_watermark_ok(WMARK_LOW) is true) until they shrink to 15MB. In my opinion, it makes more sense to have CMA take its duty to help movable allocation when U&R lower to certain zone's watermark instead of when their size become smaller than CMA.
> > Also, this is a hot path, please, make sure you're not adding much overhead.
> I would like to take more thought.

Got it, thank you for the explanation!

How about the following approach (completely untested)?

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6da423ec356f..4b50f497c09d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2279,12 +2279,13 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype,
 	if (IS_ENABLED(CONFIG_CMA)) {
 		/*
 		 * Balance movable allocations between regular and CMA areas by
-		 * allocating from CMA when over half of the zone's free memory
-		 * is in the CMA area.
+		 * allocating from CMA when over half of the zone's easily
+		 * available free memory is in the CMA area.
 		 */
 		if (alloc_flags & ALLOC_CMA &&
 		    zone_page_state(zone, NR_FREE_CMA_PAGES) >
-		    zone_page_state(zone, NR_FREE_PAGES) / 2) {
+		    (zone_page_state(zone, NR_FREE_PAGES) -
+		     zone->_watermark[WMARK_LOW]) / 2) {
 			page = __rmqueue_cma_fallback(zone, order);
 			if (page)
 				return page;

Basically the idea is to keep free space equally split between cma and non-cma areas.
Will it work for you?

Thanks!