[PATCH v8 14/30] mm: zswap: reject high-order swap cache allocations backed by zswap

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

zswap stores swapped-out pages one at a time, so a range of swap slots can
be split between zswap entries and the backing device. A high-order
swap-cache folio over such a range cannot be read back in one go: the disk
read would return stale or zero data for every slot zswap owns.

Reject that in __swap_cache_alloc(). Do the check after the folio is in the
swap cache: from that point zswap can neither start storing nor start
writing back any slot in the range, so the answer is stable. Callers
already fall back to order-0, which zswap_load() handles per page.

Export zswap_is_present() for the PMD-order swap-in users added later.

Co-developed-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 include/linux/zswap.h |  6 ++++++
 mm/swap_state.c       | 39 +++++++++++++++++++++++++--------------
 mm/zswap.c            | 12 +++++++++---
 3 files changed, 40 insertions(+), 17 deletions(-)

diff --git a/include/linux/zswap.h b/include/linux/zswap.h
index df6cafbe95dc0..94746fb71bb65 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_is_present(swp_entry_t entry, unsigned int nr);
 #else
 
 struct zswap_lruvec_state {};
@@ -73,6 +74,11 @@ static inline bool zswap_never_enabled(void)
 	return true;
 }
 
+static inline bool zswap_is_present(swp_entry_t entry, unsigned int nr)
+{
+	return false;
+}
+
 #endif
 
 #endif /* _LINUX_ZSWAP_H */
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 625c185a1ca4d..251b48b2c60e1 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -12,6 +12,7 @@
 #include <linux/kernel_stat.h>
 #include <linux/mempolicy.h>
 #include <linux/swap.h>
+#include <linux/zswap.h>
 #include <linux/leafops.h>
 #include <linux/init.h>
 #include <linux/pagemap.h>
@@ -466,26 +467,27 @@ static struct folio *__swap_cache_alloc(struct swap_cluster_info *ci,
 	__swap_cache_do_add_folio(ci, folio, entry);
 	spin_unlock(&ci->lock);
 
+	/*
+	 * Now that the folio is in the swap cache, zswap can no longer start
+	 * storing or writing back any slot in the range, so this is a stable
+	 * answer. Reject a high-order allocation over a range that already
+	 * has per-page zswap entries.
+	 */
+	if (order && zswap_is_present(entry, nr_pages)) {
+		err = -EBUSY;
+		goto delete_folio;
+	}
+
 	if (mem_cgroup_swapin_charge_folio(folio, memcg_id,
 					   vmf ? vmf->vma->vm_mm : NULL, gfp)) {
-		spin_lock(&ci->lock);
-		__swap_cache_do_del_folio(ci, folio, entry, shadow);
-		spin_unlock(&ci->lock);
-		folio_unlock(folio);
-		/* nr_pages refs from swap cache, 1 from allocation */
-		folio_put_refs(folio, nr_pages + 1);
+		err = -ENOMEM;
 		count_mthp_stat(order, MTHP_STAT_SWPIN_FALLBACK_CHARGE);
-		return ERR_PTR(-ENOMEM);
+		goto delete_folio;
 	}
 
 	if (order > 1 && folio_memcg_alloc_deferred(folio)) {
-		spin_lock(&ci->lock);
-		__swap_cache_do_del_folio(ci, folio, entry, shadow);
-		spin_unlock(&ci->lock);
-		folio_unlock(folio);
-		/* nr_pages refs from swap cache, 1 from allocation */
-		folio_put_refs(folio, nr_pages + 1);
-		return ERR_PTR(-ENOMEM);
+		err = -ENOMEM;
+		goto delete_folio;
 	}
 
 	/* memsw uncharges swap when folio is added to swap cache */
@@ -499,6 +501,15 @@ static struct folio *__swap_cache_alloc(struct swap_cluster_info *ci,
 	/* Caller will initiate read into locked new_folio */
 	folio_add_lru(folio);
 	return folio;
+
+delete_folio:
+	spin_lock(&ci->lock);
+	__swap_cache_do_del_folio(ci, folio, entry, shadow);
+	spin_unlock(&ci->lock);
+	folio_unlock(folio);
+	/* nr_pages refs from swap cache, 1 from allocation */
+	folio_put_refs(folio, nr_pages + 1);
+	return ERR_PTR(err);
 }
 
 /**
diff --git a/mm/zswap.c b/mm/zswap.c
index 507f2d19fd2a4..f5b5827d30084 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1587,11 +1587,14 @@ bool zswap_store(struct folio *folio)
  * @entry: base swap entry of the range
  * @nr: number of contiguous slots to check
  *
- * Context: The caller must keep the range pinned, otherwise the answer can
- * change under it.
+ * Context: Unless the caller keeps the range pinned, the answer is only a
+ * hint: zswap can store or write back a slot right after this returns. A
+ * caller that needs a stable answer must pin the range first, as
+ * __swap_cache_alloc() does by inserting the folio into the swap cache before
+ * asking.
  * Return: true if at least one slot in the range is in zswap.
  */
-static bool zswap_is_present(swp_entry_t entry, unsigned int nr)
+bool zswap_is_present(swp_entry_t entry, unsigned int nr)
 {
 	pgoff_t offset = swp_offset(entry);
 	struct xarray *tree = swap_zswap_tree(entry);
@@ -1605,6 +1608,9 @@ static bool zswap_is_present(swp_entry_t entry, unsigned int nr)
 	 */
 	BUILD_BUG_ON(SWAPFILE_CLUSTER > ZSWAP_ADDRESS_SPACE_PAGES);
 
+	if (zswap_never_enabled())
+		return false;
+
 	return xa_find(tree, &index, offset + nr - 1, XA_PRESENT);
 }
 
-- 
2.53.0-Meta