[PATCH] page_io: return proper error codes for swap_read_folio_zeromap()

[Nhat Pham <nphamcs@gmail.com>]

Similar to zswap_load(), also return proper error codes for
swap_read_folio_zeromap():

* 0 on success. The folio is unlocked and marked up-to-date.
* -ENOENT, if the folio is entirely not zeromapped.
* -EINVAL (with the follio unlocked but not marked to date), if the
  folio is partially zeromapped. This is not supported, and will SIGBUS
  the faulting process.

This patch is purely a clean-up, and should not have any behavioral
change. It is based on (and should be applied on top of) [1].

[1]: https://lore.kernel.org/linux-mm/20250306205011.784787-1-nphamcs@gmail.com/

Suggested-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
---
 mm/page_io.c | 35 ++++++++++++++++++++++++++---------
 1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index 4bce19df557b..48ed1e810392 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -511,7 +511,23 @@ static void sio_read_complete(struct kiocb *iocb, long ret)
 	mempool_free(sio, sio_pool);
 }
 
-static bool swap_read_folio_zeromap(struct folio *folio)
+/**
+ * swap_read_folio_zeromap - check if the folio was zeromapped, and if so,
+ *                           zero-fill it.
+ * @folio: the folio.
+ *
+ * Return: 0 on success, with the folio zero-filled, unlocked, and marked
+ * up-to-date, or one of the following error codes:
+ *
+ *  -ENOENT: the folio is entirely not zeromapped. The folio remains locked.
+ *
+ *  -EINVAL: some of the subpages in the folio are zeromaped, but not all of
+ *  them. This is an error because we don't currently support a large folio
+ *  that is partially in the zeromap. The folio is unlocked, but NOT marked
+ *  up-to-date, so that an IO error is emitted (e.g. do_swap_page() will
+ *  sigbus).
+ */
+static int swap_read_folio_zeromap(struct folio *folio)
 {
 	int nr_pages = folio_nr_pages(folio);
 	struct obj_cgroup *objcg;
@@ -519,15 +535,17 @@ static bool swap_read_folio_zeromap(struct folio *folio)
 
 	/*
 	 * Swapping in a large folio that is partially in the zeromap is not
-	 * currently handled. Return true without marking the folio uptodate so
+	 * currently handled. Return -EINVAL without marking the folio uptodate so
 	 * that an IO error is emitted (e.g. do_swap_page() will sigbus).
 	 */
 	if (WARN_ON_ONCE(swap_zeromap_batch(folio->swap, nr_pages,
-			&is_zeromap) != nr_pages))
-		return true;
+			&is_zeromap) != nr_pages)) {
+		folio_unlock(folio);
+		return -EINVAL;
+	}
 
 	if (!is_zeromap)
-		return false;
+		return -ENOENT;
 
 	objcg = get_obj_cgroup_from_folio(folio);
 	count_vm_events(SWPIN_ZERO, nr_pages);
@@ -538,7 +556,8 @@ static bool swap_read_folio_zeromap(struct folio *folio)
 
 	folio_zero_range(folio, 0, folio_size(folio));
 	folio_mark_uptodate(folio);
-	return true;
+	folio_unlock(folio);
+	return 0;
 }
 
 static void swap_read_folio_fs(struct folio *folio, struct swap_iocb **plug)
@@ -635,10 +654,8 @@ void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
 	}
 	delayacct_swapin_start();
 
-	if (swap_read_folio_zeromap(folio)) {
-		folio_unlock(folio);
+	if (swap_read_folio_zeromap(folio) != -ENOENT)
 		goto finish;
-	}
 
 	if (zswap_load(folio) != -ENOENT)
 		goto finish;
-- 
2.43.5