Patch "mm/swap.c: flush lru pvecs on compound page arrival" has been added to the 4.6-stable tree

This is a note to let you know that I've just added the patch titled

mm/swap.c: flush lru pvecs on compound page arrival

to the 4.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-swap.c-flush-lru-pvecs-on-compound-page-arrival.patch
and it can be found in the queue-4.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Lukasz Odzioba <[email protected]>
Date: Fri, 24 Jun 2016 14:50:01 -0700
Subject: mm/swap.c: flush lru pvecs on compound page arrival

From: Lukasz Odzioba <[email protected]>

commit 8f182270dfec432e93fae14f9208a6b9af01009f upstream.

Currently we can have compound pages held on per cpu pagevecs, which leads to a lot of memory unavailable for reclaim when needed. In the systems with hundreads of processors it can be GBs of memory.

On of the way of reproducing the problem is to not call munmap explicitly on all mapped regions (i.e. after receiving SIGTERM). After that some pages (with THP enabled also huge pages) may end up on lru_add_pvec, example below.

void main() {
#pragma omp parallel

  {
	size_t size = 55 * 1000 * 1000; // smaller than  MEM/CPUS
	void *p = mmap(NULL, size, PROT_READ | PROT_WRITE,
		MAP_PRIVATE | MAP_ANONYMOUS , -1, 0);
	if (p != MAP_FAILED)
		memset(p, 0, size);
	//munmap(p, size); // uncomment to make the problem go away

}
}

When we run it with THP enabled it will leave significant amount of memory on lru_add_pvec. This memory will be not reclaimed if we hit OOM, so when we run above program in a loop:

	for i in `seq 100`; do ./a.out; done

many processes (95% in my case) will be killed by OOM.

The primary point of the LRU add cache is to save the zone lru_lock contention with a hope that more pages will belong to the same zone and so their addition can be batched. The huge page is already a form of batched addition (it will add 512 worth of memory in one go) so skipping the batching seems like a safer option when compared to a potential excess in the caching which can be quite large and much harder to fix because lru_add_drain_all is way to expensive and it is not really clear what would be a good moment to call it.

Similarly we can reproduce the problem on lru_deactivate_pvec by adding:
madvise(p, size, MADV_FREE); after memset.

This patch flushes lru pvecs on compound page arrival making the problem less severe - after applying it kill rate of above example drops to 0%, due to reducing maximum amount of memory held on pvec from 28MB (with THP) to 56kB per CPU.

---
 mm/swap.c |   11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

@@ -239,7 +239,7 @@ void rotate_reclaimable_page(struct page
 		get_page(page);
 		local_irq_save(flags);
 		pvec = this_cpu_ptr(&lru_rotate_pvecs);
-		if (!pagevec_add(pvec, page))
+		if (!pagevec_add(pvec, page) || PageCompound(page))
 			pagevec_move_tail(pvec);
 		local_irq_restore(flags);
 	}

@@ -295,7 +295,7 @@ void activate_page(struct page *page)
 		struct pagevec *pvec = &get_cpu_var(activate_page_pvecs);
 
 		get_page(page);
-		if (!pagevec_add(pvec, page))
+		if (!pagevec_add(pvec, page) || PageCompound(page))
 			pagevec_lru_move_fn(pvec, __activate_page, NULL);
 		put_cpu_var(activate_page_pvecs);
 	}

@@ -390,9 +390,8 @@ static void __lru_cache_add(struct page
 	struct pagevec *pvec = &get_cpu_var(lru_add_pvec);
 
 	get_page(page);
-	if (!pagevec_space(pvec))
+	if (!pagevec_add(pvec, page) || PageCompound(page))
 		__pagevec_lru_add(pvec);
-	pagevec_add(pvec, page);
 	put_cpu_var(lru_add_pvec);
 }
 

@@ -627,7 +626,7 @@ void deactivate_file_page(struct page *p
 	if (likely(get_page_unless_zero(page))) {
 		struct pagevec *pvec = &get_cpu_var(lru_deactivate_file_pvecs);
 
-		if (!pagevec_add(pvec, page))
+		if (!pagevec_add(pvec, page) || PageCompound(page))
 			pagevec_lru_move_fn(pvec, lru_deactivate_file_fn, NULL);
 		put_cpu_var(lru_deactivate_file_pvecs);
 	}

@@ -647,7 +646,7 @@ void deactivate_page(struct page *page)
 		struct pagevec *pvec = &get_cpu_var(lru_deactivate_pvecs);
 
 		get_page(page);
-		if (!pagevec_add(pvec, page))
+		if (!pagevec_add(pvec, page) || PageCompound(page))
 			pagevec_lru_move_fn(pvec, lru_deactivate_fn, NULL);
 		put_cpu_var(lru_deactivate_pvecs);
 	}

Patches currently in stable-queue which might be from [email protected] are

queue-4.6/mm-swap.c-flush-lru-pvecs-on-compound-page-arrival.patch