[PATCH v3 04/11] mm: zswap: add range lookup for large-folio swapin

[Usama Arif <usama.arif@linux.dev>]

From: Alexandre Ghiti <alexghiti@fb.com>

A large folio reaches zswap_load() only when the caller expects the
whole range to be on disk. Zswap still stores large folios as
independent order-0 entries, so reconstructing a large folio from
zswap entries would risk returning partially initialized data.

Teach zswap_load() to scan the covered range. If no slot is in zswap,
return -ENOENT so swap_read_folio() reads the backing device. If any
slot is still in zswap, fail the large-folio read so the caller can
fall back to per-page swapin.

Add zswap_range_has_entry() so PMD swap-entry consumers can make the
same range decision before attempting PMD-order swapin.

Signed-off-by: Alexandre Ghiti <alexghiti@fb.com>
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 include/linux/zswap.h |  7 +++++++
 mm/zswap.c            | 46 +++++++++++++++++++++++++++++++++----------
 2 files changed, 43 insertions(+), 10 deletions(-)

diff --git a/include/linux/zswap.h b/include/linux/zswap.h
index 30c193a1207e..de10aa528597 100644
--- a/include/linux/zswap.h
+++ b/include/linux/zswap.h
@@ -35,6 +35,7 @@ void zswap_lruvec_state_init(struct lruvec *lruvec);
 void zswap_folio_swapin(struct folio *folio);
 bool zswap_is_enabled(void);
 bool zswap_never_enabled(void);
+bool zswap_range_has_entry(swp_entry_t entry, unsigned int nr);
 #else
 
 struct zswap_lruvec_state {};
@@ -69,6 +70,12 @@ static inline bool zswap_never_enabled(void)
 	return true;
 }
 
+static inline bool zswap_range_has_entry(swp_entry_t entry,
+					 unsigned int nr)
+{
+	return false;
+}
+
 #endif
 
 #endif /* _LINUX_ZSWAP_H */
diff --git a/mm/zswap.c b/mm/zswap.c
index b5a17ea20237..89dd88a5223f 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1559,6 +1559,27 @@ bool zswap_store(struct folio *folio)
 	return ret;
 }
 
+/**
+ * zswap_range_has_entry() - is any slot in [entry, entry + nr) in zswap?
+ * @entry: base swap entry of the range
+ * @nr: number of contiguous slots to check
+ */
+bool zswap_range_has_entry(swp_entry_t entry, unsigned int nr)
+{
+	pgoff_t offset = swp_offset(entry);
+	XA_STATE(xas, swap_zswap_tree(entry), offset);
+	bool found;
+
+	if (!nr || zswap_never_enabled())
+		return false;
+
+	rcu_read_lock();
+	found = !!xas_find(&xas, offset + nr - 1);
+	rcu_read_unlock();
+
+	return found;
+}
+
 /**
  * zswap_load() - load a folio from zswap
  * @folio: folio to load
@@ -1571,10 +1592,9 @@ bool zswap_store(struct folio *folio)
  *  NOT marked up-to-date, so that an IO error is emitted (e.g. do_swap_page()
  *  will SIGBUS).
  *
- *  -EINVAL: if the swapped out content was in zswap, but the page belongs
- *  to a large folio, which is not supported by zswap. The folio is unlocked,
- *  but NOT marked up-to-date, so that an IO error is emitted (e.g.
- *  do_swap_page() will SIGBUS).
+ *  -EIO: if a slot in a large-folio range is unexpectedly still in zswap.
+ *  The folio is unlocked, but NOT marked up-to-date, so that an IO
+ *  error is emitted (e.g. do_swap_page() will SIGBUS).
  *
  *  -ENOENT: if the swapped out content was not in zswap. The folio remains
  *  locked on return.
@@ -1593,13 +1613,19 @@ int zswap_load(struct folio *folio)
 		return -ENOENT;
 
 	/*
-	 * Large folios should not be swapped in while zswap is being used, as
-	 * they are not properly handled. Zswap does not properly load large
-	 * folios, and a large folio may only be partially in zswap.
+	 * A large folio reaches zswap_load() only when its whole range is
+	 * expected to be on disk: PMD swap-entry consumers split before
+	 * calling into PMD-order swapin whenever any slot is still in zswap.
+	 * Confirm the range is entirely absent from zswap and return -ENOENT
+	 * so the caller reads it from disk; if a slot is unexpectedly still in
+	 * zswap, fail the read rather than return partially-initialized data.
 	 */
-	if (WARN_ON_ONCE(folio_test_large(folio))) {
-		folio_unlock(folio);
-		return -EINVAL;
+	if (folio_test_large(folio)) {
+		if (zswap_range_has_entry(swp, folio_nr_pages(folio))) {
+			folio_unlock(folio);
+			return -EIO;
+		}
+		return -ENOENT;
 	}
 
 	entry = xa_load(tree, offset);
-- 
2.53.0-Meta