[RFC PATCH v3 2/2] mm: vmscan: retry folios written back while isolated

[Chen Ridong]

From: Chen Ridong <chenridong@xxxxxxxxxx>

An issue was found with the following testing step:
1. Compile with CONFIG_TRANSPARENT_HUGEPAGE=y, CONFIG_LRU_GEN_ENABLED=n.
2. Mount memcg v1, and create memcg named test_memcg and set
   usage_in_bytes=2.1G, memsw.usage_in_bytes=3G.
3. Use file as swap, and create a 1G swap.
4. Allocate 2.2G anon memory in test_memcg.

It was found that:

cat memory.usage_in_bytes
2144940032
cat memory.memsw.usage_in_bytes
2255056896

free -h
              total        used        free
Mem:           31Gi       2.1Gi        27Gi
Swap:         1.0Gi       618Mi       405Mi

As shown above, the test_memcg used about 100M swap, but 600M+ swap memory
was used, which means that 500M may be wasted because other memcgs can not
use these swap memory.

It can be explained as follows:
1. When entering shrink_inactive_list, it isolates folios from lru from
   tail to head. If it just takes folioN from lru(make it simple).

   inactive lru: folio1<->folio2<->folio3...<->folioN-1
   isolated list: folioN

2. In shrink_page_list function, if folioN is THP(2M), it may be splited
   and added to swap cache folio by folio. After adding to swap cache,
   it will submit io to writeback folio to swap, which is asynchronous.
   When shrink_page_list is finished, the isolated folios list will be
   moved back to the head of inactive lru. The inactive lru may just look
   like this, with 512 filioes have been move to the head of inactive lru.

   folioN512<->folioN511<->...filioN1<->folio1<->folio2...<->folioN-1

   It committed io from folioN1 to folioN512, the later folios committed
   was added to head of the 'ret_folios' in the shrink_page_list function.
   As a result, the order was shown as folioN512->folioN511->...->folioN1.

3. When folio writeback io is completed, the folio may be rotated to tail
   of the lru one by one. It's assumed that filioN1,filioN2, ...,filioN512
   are completed in order(commit io in this order), and they are rotated to
   the tail of the LRU in order (filioN1<->...folioN511<->folioN512).
   Therefore, those folios that are tail of the lru will be reclaimed as
   soon as possible.

   folio1<->folio2<->...<->folioN-1<->filioN1<->...folioN511<->folioN512

4. However, shrink_page_list and folio writeback are asynchronous. If THP
   is splited, shrink_page_list loops at least 512 times, which means that
   shrink_page_list is not completed but some folios writeback have been
   completed, and this may lead to failure to rotate these folios to the
   tail of lru. The lru may look likes as below:

   folioN50<->folioN49<->...filioN1<->folio1<->folio2...<->folioN-1<->
   folioN51<->folioN52<->...folioN511<->folioN512

   Although those folios (N1-N50) have been finished writing back, they
   are still at the head of the lru. This is because their writeback_end
   occurred while it were still looping in shrink_folio_list(), causing
   folio_end_writeback()'s folio_rotate_reclaimable() to fail in moving
   these folios, which are not in the LRU but still in the 'folio_list',
   to the tail of the LRU.
   When isolating folios from lru, it scans from tail to head, so it is
   difficult to scan those folios again.

This issue is fixed when CONFIG_LRU_GEN_ENABLED is enabled with the
commit 359a5e1416ca ("mm: multi-gen LRU: retry folios written back while
isolated"). This issue should be fixed for active/inactive lru in the
same way.

Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
---
 mm/vmscan.c | 35 +++++++++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index af1ff76f83e7..1f0d194f8b2f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1949,6 +1949,25 @@ static int current_may_throttle(void)
 	return !(current->flags & PF_LOCAL_THROTTLE);
 }
 
+static inline void acc_reclaimed_stat(struct reclaim_stat *stat,
+		struct reclaim_stat *curr)
+{
+	int i;
+
+	stat->nr_dirty += curr->nr_dirty;
+	stat->nr_unqueued_dirty += curr->nr_unqueued_dirty;
+	stat->nr_congested += curr->nr_congested;
+	stat->nr_writeback += curr->nr_writeback;
+	stat->nr_immediate += curr->nr_immediate;
+	stat->nr_pageout += curr->nr_pageout;
+	stat->nr_ref_keep += curr->nr_ref_keep;
+	stat->nr_unmap_fail += curr->nr_unmap_fail;
+	stat->nr_lazyfree_fail += curr->nr_lazyfree_fail;
+	stat->nr_demoted += curr->nr_demoted;
+	for (i = 0; i < ANON_AND_FILE; i++)
+		stat->nr_activate[i] = curr->nr_activate[i];
+}
+
 /*
  * shrink_inactive_list() is a helper for shrink_node().  It returns the number
  * of reclaimed pages
@@ -1958,14 +1977,16 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
 		enum lru_list lru)
 {
 	LIST_HEAD(folio_list);
+	LIST_HEAD(clean_list);
 	unsigned long nr_scanned;
 	unsigned int nr_reclaimed = 0;
 	unsigned long nr_taken;
-	struct reclaim_stat stat;
+	struct reclaim_stat stat, curr;
 	bool file = is_file_lru(lru);
 	enum vm_event_item item;
 	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
 	bool stalled = false;
+	bool skip_retry = false;
 
 	while (unlikely(too_many_isolated(pgdat, file, sc))) {
 		if (stalled)
@@ -1999,10 +2020,20 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
 	if (nr_taken == 0)
 		return 0;
 
-	nr_reclaimed = shrink_folio_list(&folio_list, pgdat, sc, &stat, false);
+	memset(&stat, 0, sizeof(stat));
+retry:
+	nr_reclaimed += shrink_folio_list(&folio_list, pgdat, sc, &curr, false);
+	find_folios_written_back(&folio_list, &clean_list, skip_retry);
+	acc_reclaimed_stat(&stat, &curr);
 
 	spin_lock_irq(&lruvec->lru_lock);
 	move_folios_to_lru(lruvec, &folio_list);
+	if (!list_empty(&clean_list)) {
+		list_splice_init(&clean_list, &folio_list);
+		skip_retry = true;
+		spin_unlock_irq(&lruvec->lru_lock);
+		goto retry;
+	}
 
 	__mod_lruvec_state(lruvec, PGDEMOTE_KSWAPD + reclaimer_offset(),
 					stat.nr_demoted);
-- 
2.34.1