[Devel] [PATCH rh7 7/7] tswap: do not writeback pages on reclaim

[Vladimir Davydov]

Writeback slows down tswap reclaim significantly, resulting in
unpredictable stalls. Generic reclaim path takes into account a lot of
stats in order to avoid such stalls or handle them out to kswapd. We
cannot afford it, because we are outside vmscan. Instead, let us simply
move evicted tswap pages back to swapcache, marking them dirty. To avoid
ping-ponging pages from swapcache to tswap and back again, we make tswap
store avoid using emergency reserves so that a store is very likely to
fail under hard memory pressure.

Signed-off-by: Vladimir Davydov <vdavydov at parallels.com>
---
 mm/tswap.c | 20 ++++++--------------
 1 file changed, 6 insertions(+), 14 deletions(-)

diff --git a/mm/tswap.c b/mm/tswap.c
index 4b8cef9ed0f8..cdaa98ad2e2d 100644
--- a/mm/tswap.c
+++ b/mm/tswap.c
@@ -11,7 +11,6 @@
 #include <linux/swap.h>
 #include <linux/swapops.h>
 #include <linux/pagemap.h>
-#include <linux/writeback.h>
 #include <linux/shrinker.h>
 #include <linux/frontswap.h>
 
@@ -103,15 +102,12 @@ static unsigned long tswap_shrink_count(struct shrinker *shrink,
 	return tswap_lru_node[sc->nid].nr_items;
 }
 
-static int tswap_writeback_page(struct page *page)
+static int tswap_evict_page(struct page *page)
 {
 	struct address_space *swapper_space;
 	struct page *found_page;
 	swp_entry_t entry;
 	int err;
-	struct writeback_control wbc = {
-		.sync_mode = WB_SYNC_NONE,
-	};
 
 	BUG_ON(!PageLocked(page));
 
@@ -196,18 +192,12 @@ retry:
 
 	lru_cache_add_anon(page);
 	SetPageUptodate(page);
-
-	/* move it to the tail of the inactive list after end_writeback */
-	SetPageReclaim(page);
-
-	/* start writeback; unlocks the page */
-	__swap_writepage(page, &wbc, end_swap_bio_write);
+	SetPageDirty(page);
 	return 0;
 
 out_free_swapcache:
 	swapcache_free(entry, NULL);
 out:
-	unlock_page(page);
 	return err;
 }
 
@@ -235,9 +225,10 @@ static unsigned long tswap_shrink_scan(struct shrinker *shrink,
 		get_page(page);
 		spin_unlock(&tswap_lock);
 
-		if (tswap_writeback_page(page) == 0)
+		if (tswap_evict_page(page) == 0)
 			nr_reclaimed++;
 
+		unlock_page(page);
 		put_page(page);
 
 		cond_resched();
@@ -277,7 +268,8 @@ static int tswap_frontswap_store(unsigned type, pgoff_t offset,
 	if (cache_page)
 		goto copy;
 
-	cache_page = alloc_page(__GFP_HIGHMEM | __GFP_NORETRY | __GFP_NOWARN);
+	cache_page = alloc_page(__GFP_HIGHMEM | __GFP_NORETRY |
+				__GFP_NOMEMALLOC | __GFP_NOWARN);
 	if (!cache_page)
 		return -1;
 
-- 
2.1.4