[PATCH v2 1/1] mm/swap.c: flush lru pvecs on compound page arrival

[Lukasz Odzioba]

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.

Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
Signed-off-by: Lukasz Odzioba <lukasz.odzioba@xxxxxxxxx>

---
v2: Flush all pvecs instead of just lru_add_pvec
---
 mm/swap.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index 59f5faf..90530ff 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -242,7 +242,7 @@ void rotate_reclaimable_page(struct page *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);
 	}
@@ -296,7 +296,7 @@ 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);
 	}
@@ -391,9 +391,8 @@ static void __lru_cache_add(struct page *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);
 }
 
@@ -628,7 +627,7 @@ void deactivate_file_page(struct page *page)
 	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);
 	}
@@ -648,7 +647,7 @@ 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);
 	}
-- 
1.8.3.1