[RFC PATCH 06/10] mm/zswap: implement ghost-to-physical writeback for backend switching

[Baoquan He <baoquan.he@linux.dev>]

Add zswap_writeback_ghost() to handle zswap writeback when the
backing swap device is a ghost swapfile (SWP_GHOST).  Previously,
writeback for ghost entries was rejected with -EINVAL.

The new writeback path:
1. Allocates a physical swap slot from any real swap device via
   alloc_phys_swap_slot().
2. Creates a swap-cache folio attached to the ghost swap entry.
3. Decompresses the zswap entry into the folio.
4. Stores the ghost->physical mapping in the ghost's redirect_xa.
5. Temporarily redirects folio->swap to the physical entry and
   calls __swap_writepage() to write to the physical device.
6. On success, the xarray mapping persists.  When the swap-cache
   folio is later removed, __swap_cache_do_del_folio() restores
   the Redirect entry from the xarray.

This enables the backend switch from zswap (compressed in memory)
to physical swap (on disk) without changing the PTE — the PTE
continues to point to the ghost swap entry, and the swapin path
forwards the I/O via the Redirect entry.

Modify zswap_writeback_entry() to call zswap_writeback_ghost()
for ghost-backed entries instead of returning -EINVAL.

Signed-off-by: Baoquan He <baoquan.he@linux.dev>
---
 mm/zswap.c | 96 +++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 91 insertions(+), 5 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index bdf8283fd880..2137e8f14413 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -978,6 +978,90 @@ static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio)
  * the swap cache, the compressed version stored by zswap can be
  * freed.
  */
+/*
+ * Ghost swapfile writeback: allocate a physical swap slot, decompress
+ * the zswap entry into a swap-cache folio, write it to the physical
+ * device via __swap_writepage, and set up a Redirect entry on the
+ * ghost swap table so future swapins are forwarded to the physical
+ * device.
+ */
+static int zswap_writeback_ghost(struct zswap_entry *entry,
+				 swp_entry_t swpentry,
+				 struct swap_info_struct *ghost_si,
+				 pgoff_t offset)
+{
+	swp_entry_t phys_entry;
+	struct folio *folio;
+	struct mempolicy *mpol;
+	swp_entry_t saved_entry;
+	int ret = 0;
+
+	/* Allocate a physical swap slot from a real device */
+	phys_entry = alloc_phys_swap_slot();
+	if (!phys_entry.val)
+		return -ENOMEM;
+
+	/* Create a swap-cache folio attached to the ghost swap entry */
+	mpol = get_task_policy(current);
+	folio = swap_cache_alloc_folio(swpentry, GFP_KERNEL, BIT(0), NULL, mpol,
+				       NO_INTERLEAVE_INDEX, NULL);
+	if (IS_ERR(folio)) {
+		ret = PTR_ERR(folio);
+		goto free_phys;
+	}
+
+	/* Decompress zswap data into the folio */
+	if (!zswap_decompress(entry, folio)) {
+		ret = -EIO;
+		goto del_folio;
+	}
+
+	count_vm_event(ZSWPWB);
+	if (entry->objcg)
+		count_objcg_events(entry->objcg, ZSWPWB, 1);
+
+	zswap_entry_free(entry);
+
+	folio_mark_uptodate(folio);
+	folio_set_reclaim(folio);
+
+	/*
+	 * Store the redirect mapping in the xarray so it persists
+	 * across swap-cache PFN cycles.  The swap table currently
+	 * holds a PFN (the swap cache folio); when the folio is
+	 * eventually removed from swap cache, the Redirect entry
+	 * will be restored from the xarray.
+	 */
+	xa_store(&ghost_si->redirect_xa, offset,
+		 xa_mk_value(phys_entry.val), GFP_ATOMIC);
+
+	/*
+	 * Write the folio to the physical swap device.  Temporarily
+	 * redirect folio->swap to the physical entry so
+	 * __swap_writepage uses the correct bdev and sector.
+	 */
+	saved_entry = folio->swap;
+	folio->swap = phys_entry;
+	ret = __swap_writepage(folio, NULL);
+	folio->swap = saved_entry;
+
+	if (ret) {
+		xa_erase(&ghost_si->redirect_xa, offset);
+		goto del_folio;
+	}
+
+	folio_put(folio);
+	return 0;
+
+del_folio:
+	swap_cache_del_folio(folio);
+	folio_unlock(folio);
+	folio_put(folio);
+free_phys:
+	free_phys_swap_slot(phys_entry);
+	return ret;
+}
+
 static int zswap_writeback_entry(struct zswap_entry *entry,
 				 swp_entry_t swpentry)
 {
@@ -994,11 +1078,6 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	if (!si)
 		return -ENOENT;
 
-	if (si->flags & SWP_GHOST) {
-		put_swap_device(si);
-		return -EINVAL;
-	}
-
 	/*
 	 * Verify the swap table Pointer entry still references this
 	 * zswap_entry. If the entry was invalidated or replaced by a
@@ -1015,6 +1094,13 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
 	}
 	spin_unlock(&ci->lock);
 
+	/* Ghost swapfile: write back to a physical swap device */
+	if (si->flags & SWP_GHOST) {
+		ret = zswap_writeback_ghost(entry, swpentry, si, offset);
+		put_swap_device(si);
+		return ret;
+	}
+
 	mpol = get_task_policy(current);
 	folio = swap_cache_alloc_folio(swpentry, GFP_KERNEL, BIT(0), NULL, mpol,
 				       NO_INTERLEAVE_INDEX, NULL);
-- 
2.54.0