[PATCH v3 mm-hotfixes] mm/zswap: fix inconsistency when zswap_store_page() fails

[Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>]

Commit b7c0ccdfbafd ("mm: zswap: support large folios in zswap_store()")
skips charging any zswap entries when it failed to zswap the entire
folio.

However, when some base pages are zswapped but it failed to zswap
the entire folio, the zswap operation is rolled back.
When freeing zswap entries for those pages, zswap_entry_free() uncharges
the zswap entries that were not previously charged, causing zswap charging
to become inconsistent.

This inconsistency triggers two warnings with following steps:
  # On a machine with 64GiB of RAM and 36GiB of zswap
  $ stress-ng --bigheap 2 # wait until the OOM-killer kills stress-ng
  $ sudo reboot

  The two warnings are:
    in mm/memcontrol.c:163, function obj_cgroup_release():
      WARN_ON_ONCE(nr_bytes & (PAGE_SIZE - 1));

    in mm/page_counter.c:60, function page_counter_cancel():
      if (WARN_ONCE(new < 0, "page_counter underflow: %ld nr_pages=%lu\n",
	  new, nr_pages))

zswap_stored_pages also becomes inconsistent in the same way.

As suggested by Kanchana, increment zswap_stored_pages and charge zswap
entries within zswap_store_page() when it succeeds. This way,
zswap_entry_free() will decrement the counter and uncharge the entries
when it failed to zswap the entire folio.

While this could potentially be optimized by batching objcg charging
and incrementing the counter, let's focus on fixing the bug this time
and leave the optimization for later after some evaluation.

After resolving the inconsistency, the warnings disappear.

Fixes: b7c0ccdfbafd ("mm: zswap: support large folios in zswap_store()")
Cc: stable@xxxxxxxxxxxxxxx
Co-developed-by: Kanchana P Sridhar <kanchana.p.sridhar@xxxxxxxxx>
Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@xxxxxxxxx>
Signed-off-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
---

v2 -> v3:
  - Adjusted Kanchana's feedback:
    - Fixed inconsistency in zswap_stored_pages
    - Now objcg charging and incrementing zswap_store_pages is done
      within zswap_stored_pages, one by one

 mm/zswap.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 6504174fbc6a..f0bd962bffd5 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1504,11 +1504,14 @@ static ssize_t zswap_store_page(struct page *page,
 	entry->pool = pool;
 	entry->swpentry = page_swpentry;
 	entry->objcg = objcg;
+	if (objcg)
+		obj_cgroup_charge_zswap(objcg, entry->length);
 	entry->referenced = true;
 	if (entry->length) {
 		INIT_LIST_HEAD(&entry->lru);
 		zswap_lru_add(&zswap_list_lru, entry);
 	}
+	atomic_long_inc(&zswap_stored_pages);
 
 	return entry->length;
 
@@ -1526,7 +1529,6 @@ bool zswap_store(struct folio *folio)
 	struct obj_cgroup *objcg = NULL;
 	struct mem_cgroup *memcg = NULL;
 	struct zswap_pool *pool;
-	size_t compressed_bytes = 0;
 	bool ret = false;
 	long index;
 
@@ -1569,15 +1571,11 @@ bool zswap_store(struct folio *folio)
 		bytes = zswap_store_page(page, objcg, pool);
 		if (bytes < 0)
 			goto put_pool;
-		compressed_bytes += bytes;
 	}
 
-	if (objcg) {
-		obj_cgroup_charge_zswap(objcg, compressed_bytes);
+	if (objcg)
 		count_objcg_events(objcg, ZSWPOUT, nr_pages);
-	}
 
-	atomic_long_add(nr_pages, &zswap_stored_pages);
 	count_vm_events(ZSWPOUT, nr_pages);
 
 	ret = true;
-- 
2.47.1