[PATCH v6.12 hotfix] 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()")
mistakenly skipped charging any zswapped pages when a single call to
zswap_store_page() failed, even if some pages in the folio are
successfully stored in zswap.

Making things worse, these not-charged pages are uncharged in
zswap_entry_free(), making zswap charging inconsistent.

This inconsistency triggers two warnings when following these 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))

Charge zswapped pages even if some pages of the folio are not zswapped.
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>
---
 mm/zswap.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 6504174fbc6a..92752cd05c75 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1568,20 +1568,20 @@ 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);
-		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:
+	if (objcg) {
+		obj_cgroup_charge_zswap(objcg, compressed_bytes);
+		count_objcg_events(objcg, ZSWPOUT, nr_pages);
+	}
 put_pool:
 	zswap_pool_put(pool);
 put_objcg:
-- 
2.47.1