[PATCH v2 mm-hotfixes] mm/zswap: fix inconsistent charging when zswap_store_page() fails

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

Commit b7c0ccdfbafd ("mm: zswap: support large folios in zswap_store()")
skips charging any zswapped base pages 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 pages 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

  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))

While objcg events should only be accounted for when the entire folio is
zswapped, objcg charging should be performed regardlessly.
Fix accordingly.

After resolving the inconsistency, these warnings disappear.

Fixes: b7c0ccdfbafd ("mm: zswap: support large folios in zswap_store()")
Cc: stable@vger.kernel.org
Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
---

v1->v2:

 Fixed objcg events being accounted for on zswap failure.

 Fixed the incorrect description. I misunderstood that the base pages are
 going to be stored in zswap, but their zswap entries are freed immediately.

 Added a comment on why it charges pages that are going to be removed
 from zswap.

 mm/zswap.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 6504174fbc6a..10b30ac46deb 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1568,20 +1568,26 @@ bool zswap_store(struct folio *folio)
 
 		bytes = zswap_store_page(page, objcg, pool);
 		if (bytes < 0)
-			goto put_pool;
+			goto charge_zswap;
 		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;
 
+charge_zswap:
+	/*
+	 * Charge zswapped pages even when it failed to zswap the entire folio,
+	 * because zswap_entry_free() will uncharge them anyway.
+	 * Otherwise zswap charging will become inconsistent.
+	 */
+	if (objcg)
+		obj_cgroup_charge_zswap(objcg, compressed_bytes);
 put_pool:
 	zswap_pool_put(pool);
 put_objcg:
-- 
2.47.1