[PATCH v4 06/11] mm, swap: write back vswap zswap entries to physical swap

[Nhat Pham]

Add support for writing back zswap-backed vswap entries to physical
swap. The mechanism mirrors the existing zswap writeback path, except
the backing physical slot is allocated on demand at writeback time
rather than already being pinned by the PTE.

The zswap shrinker no longer skips vswap entries, unless we are out of
physical swap space.

Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
---
 mm/zswap.c | 67 +++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 46 insertions(+), 21 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index f16c0b44b5d5..70ad8010f18a 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1007,12 +1007,13 @@ static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio)
 static int zswap_writeback_entry(struct zswap_entry *entry,
 				 swp_entry_t swpentry)
 {
-	struct xarray *tree;
 	pgoff_t offset = swp_offset(swpentry);
 	struct folio *folio;
 	struct mempolicy *mpol;
 	struct swap_info_struct *si;
 	struct swap_io_ctx ctx = {};
+	swp_entry_t phys = {};
+	bool is_vswap;
 	int ret = 0;
 
 	/* try to allocate swap cache folio */
@@ -1020,12 +1021,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	if (!si)
 		return -EEXIST;
 
-	/* Vswap entries have no physical backing to write to. */
-	if (swap_is_vswap(si)) {
-		put_swap_device(si);
-		return -EINVAL;
-	}
-
+	is_vswap = swap_is_vswap(si);
 	mpol = get_task_policy(current);
 	folio = swap_cache_alloc_folio(swpentry, GFP_KERNEL, BIT(0), NULL, mpol,
 				       NO_INTERLEAVE_INDEX);
@@ -1044,24 +1040,44 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	/*
 	 * folio is locked, and the swapcache is now secured against
 	 * concurrent swapping to and from the slot, and concurrent
-	 * swapoff so we can safely dereference the zswap tree here.
+	 * swapoff so we can safely dereference the zswap tree (or vswap
+	 * vtable) here.
 	 * Verify that the swap entry hasn't been invalidated and recycled
 	 * behind our backs, to avoid overwriting a new swap folio with
 	 * old compressed data. Only when this is successful can the entry
 	 * be dereferenced.
 	 */
-	tree = swap_zswap_tree(swpentry);
-	if (entry != xa_load(tree, offset)) {
+	if (entry != zswap_entry_load(swpentry)) {
 		ret = -ENOMEM;
 		goto out;
 	}
 
+	if (is_vswap) {
+		/*
+		 * Allocate physical backing before decompress so a failure
+		 * wastes no work.
+		 */
+		phys = folio_realloc_swap(folio);
+		if (!phys.val) {
+			ret = -ENOMEM;
+			goto out;
+		}
+	}
+
 	if (!zswap_decompress(entry, folio)) {
 		ret = -EIO;
+		/*
+		 * The phys allocation above took the entry out of the vtable.
+		 * Restore the zswap entry to the vtable, which also frees the
+		 * allocated physical swap space.
+		 */
+		if (is_vswap)
+			vswap_zswap_store(swpentry, entry);
 		goto out;
 	}
 
-	xa_erase(tree, offset);
+	if (!is_vswap)
+		xa_erase(swap_zswap_tree(swpentry), offset);
 
 	count_vm_event(ZSWPWB);
 	if (entry->objcg)
@@ -1076,7 +1092,10 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	folio_set_reclaim(folio);
 
 	/* start writeback */
-	__swap_writepage(&ctx, folio, folio->swap);
+	if (is_vswap)
+		__swap_writepage(&ctx, folio, phys);
+	else
+		__swap_writepage(&ctx, folio, folio->swap);
 	swap_write_submit(&ctx);
 
 out:
@@ -1091,6 +1110,15 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 /*********************************
 * shrinker functions
 **********************************/
+/*
+ * vswap zswap entries get a physical slot allocated on demand at writeback
+ * time. Skip the shrinker when none is available.
+ */
+static bool zswap_writeback_possible(void)
+{
+	return !vswap_is_enabled() || get_nr_swap_pages() > 0;
+}
+
 /*
  * The dynamic shrinker is modulated by the following factors:
  *
@@ -1228,7 +1256,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
 	if (!zswap_shrinker_enabled || !mem_cgroup_zswap_writeback_enabled(memcg))
 		return 0;
 
-	if (vswap_is_enabled())
+	if (!zswap_writeback_possible())
 		return 0;
 
 	/*
@@ -1314,7 +1342,8 @@ static struct shrinker *zswap_alloc_shrinker(void)
  * were scanned but none could be written back, or -ENOENT if @memcg has
  * writeback disabled, is a zombie cgroup, or has empty zswap LRUs.
  *
- * Also returns -ENOENT when vswap is enabled.
+ * Also returns -ENOENT when vswap is enabled and there is no physical
+ * swap to write back to.
  */
 static int shrink_memcg(struct mem_cgroup *memcg)
 {
@@ -1323,7 +1352,7 @@ static int shrink_memcg(struct mem_cgroup *memcg)
 	if (!mem_cgroup_zswap_writeback_enabled(memcg))
 		return -ENOENT;
 
-	if (vswap_is_enabled())
+	if (!zswap_writeback_possible())
 		return -ENOENT;
 
 	/*
@@ -1354,11 +1383,7 @@ static void shrink_worker(struct work_struct *w)
 	int ret, failures = 0, attempts = 0;
 	unsigned long thr;
 
-	/*
-	 * When vswap is enabled, zswap entries are almost all vswap backed,
-	 * with no slot to write back to.
-	 */
-	if (vswap_is_enabled())
+	if (!zswap_writeback_possible())
 		return;
 
 	/* Reclaim down to the accept threshold */
@@ -1439,7 +1464,7 @@ static void shrink_worker(struct work_struct *w)
 			break;
 resched:
 		cond_resched();
-	} while (zswap_total_pages() > thr);
+	} while (zswap_total_pages() > thr && zswap_writeback_possible());
 }
 
 /*********************************
-- 
2.53.0-Meta