[RFC PATCH 09/10] mm/swapfile: implement swap_ghost_extend_max() for dynamic growth

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

Allocate additional physical pages and map them at the end of the
already-reserved sparse vmalloc area via vm_area_map_pages(). The flat
cluster_info pointer never moves — new clusters appear contiguously at
higher indices. New clusters are fully initialized (spinlocks, list
heads, trailing BAD mask) before si->max is published via WRITE_ONCE.

Page-pointer tracking in cluster_vm->pages is reallocated to accommodate
additional entries. Returns -E2BIG if the requested size exceeds the
reserved VA range. Extension is serialized by swapon_mutex.

Signed-off-by: Baoquan He <baoquan.he@linux.dev>
---
 mm/swapfile.c | 110 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 110 insertions(+)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index da085a75d7b5..8bf336678887 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -58,6 +58,8 @@ static void move_cluster(struct swap_info_struct *si,
 			 enum swap_cluster_flags new_flags);
 #ifdef CONFIG_ZSWAP
 #define GHOST_MAX_CLUSTER_BYTES (64UL * 1024 * 1024)
+static int swap_ghost_extend_max(struct swap_info_struct *si,
+				 unsigned int new_maxpages);
 #endif
 
 /*
@@ -4247,6 +4249,114 @@ swp_entry_t ghost_redirect_to_phys(struct swap_info_struct *si, pgoff_t offset)
 
 	return (swp_entry_t){};
 }
+
+/*
+ * swap_ghost_extend_max — Extend an active ghost swap device.
+ * @si:  ghost device (SWP_GHOST set)
+ * @new: new max pages (> si->max)
+ *
+ * Allocates physical pages and maps them at the end of the reserved
+ * sparse vmalloc area.  si->cluster_info never moves.  New clusters
+ * are fully initialized before si->max is published via WRITE_ONCE.
+ *
+ * Returns 0, -ENOMEM, -EINVAL, or -E2BIG.  Caller holds swapon_mutex.
+ */
+static int swap_ghost_extend_max(struct swap_info_struct *si,
+				 unsigned int new)
+{
+	unsigned long old_max = si->max, old_nr, new_nr;
+	unsigned long old_bytes, new_bytes, nr_new_pg;
+	struct page **new_pglist;
+	unsigned long i;
+
+	if (!(si->flags & SWP_GHOST) ||
+	    !(si->flags & (SWP_USED | SWP_WRITEOK)) ||
+	    new <= si->max)
+		return -EINVAL;
+
+	old_nr = DIV_ROUND_UP(si->max, SWAPFILE_CLUSTER);
+	new_nr = DIV_ROUND_UP(new, SWAPFILE_CLUSTER);
+	old_bytes = ALIGN(old_nr * sizeof(struct swap_cluster_info), PAGE_SIZE);
+	new_bytes = ALIGN(new_nr * sizeof(struct swap_cluster_info), PAGE_SIZE);
+	nr_new_pg = (new_bytes - old_bytes) / PAGE_SIZE;
+
+	if (old_bytes + nr_new_pg * PAGE_SIZE > si->cluster_vm->size) {
+		pr_warn("ghost: extend to %u exceeds reserved VA (%lu bytes)\n",
+			new, si->cluster_vm->size);
+		return -E2BIG;
+	}
+
+	new_pglist = kvmalloc_array(si->cluster_vm->nr_pages + nr_new_pg,
+				    sizeof(struct page *), GFP_KERNEL);
+	if (!new_pglist)
+		return -ENOMEM;
+	if (si->cluster_vm->pages) {
+		memcpy(new_pglist, si->cluster_vm->pages,
+		       si->cluster_vm->nr_pages * sizeof(struct page *));
+		kvfree(si->cluster_vm->pages);
+	}
+	si->cluster_vm->pages = (void *)new_pglist;
+
+	for (i = 0; i < nr_new_pg; i++) {
+		struct page *page = alloc_page(GFP_KERNEL | __GFP_ZERO);
+		if (!page) {
+			while (i-- > 0)
+				__free_page(new_pglist[si->cluster_vm->nr_pages + i]);
+			return -ENOMEM;
+		}
+		new_pglist[si->cluster_vm->nr_pages + i] = page;
+	}
+
+	if (vm_area_map_pages(si->cluster_vm,
+			      (unsigned long)si->cluster_vm->addr + old_bytes,
+			      (unsigned long)si->cluster_vm->addr + new_bytes,
+			      &new_pglist[si->cluster_vm->nr_pages])) {
+		for (i = 0; i < nr_new_pg; i++)
+			__free_page(new_pglist[si->cluster_vm->nr_pages + i]);
+		return -ENOMEM;
+	}
+	si->cluster_vm->nr_pages += nr_new_pg;
+
+	for (i = old_nr; i < new_nr; i++) {
+		struct swap_cluster_info *ci = &si->cluster_info[i];
+		spin_lock_init(&ci->lock);
+		INIT_LIST_HEAD(&ci->list);
+	}
+
+	if (new % SWAPFILE_CLUSTER) {
+		unsigned long base = (new - 1) / SWAPFILE_CLUSTER *
+				    SWAPFILE_CLUSTER;
+		unsigned long idx = base / SWAPFILE_CLUSTER;
+		struct swap_cluster_info *ci = &si->cluster_info[idx];
+
+		if (!ci->count && swap_cluster_alloc_table(ci, GFP_KERNEL)) {
+			for (i = 0; i < nr_new_pg; i++)
+				__free_page(new_pglist[si->cluster_vm->nr_pages + i]);
+			return -ENOMEM;
+		}
+		spin_lock(&ci->lock);
+		for (i = new % SWAPFILE_CLUSTER; i < SWAPFILE_CLUSTER; i++)
+			__swap_table_set(ci, i, SWP_TB_BAD);
+		spin_unlock(&ci->lock);
+	}
+
+	WRITE_ONCE(si->max, new);
+
+	for (i = old_nr; i < new_nr; i++) {
+		struct swap_cluster_info *ci = &si->cluster_info[i];
+		ci->flags = CLUSTER_FLAG_FREE;
+		spin_lock(&si->lock);
+		list_add_tail(&ci->list, &si->free_clusters);
+		spin_unlock(&si->lock);
+	}
+
+	si->pages += new - old_max;
+	atomic_long_add(new - old_max, &nr_swap_pages);
+	spin_lock(&swap_lock);
+	total_swap_pages += new - old_max;
+	spin_unlock(&swap_lock);
+	return 0;
+}
 #endif /* CONFIG_ZSWAP */
 
 static int __init swapfile_init(void)
-- 
2.54.0