mm/vmalloc: avoid iterating over per CPU vmap blocks twice

_vunmap_aliases() walks the per CPU xarrays to find partially unmapped
blocks and then walks the per cpu free lists to purge fragmented blocks.

Arguably that's waste of CPU cycles and cache lines as the full xarray
walk already touches every block.

Avoid this double iteration:

  - Split out the code to purge one block and the code to free the local
    purge list into helper functions.

  - Try to purge the fragmented blocks in the xarray walk before looking at
    their dirty space.

Link: https://lkml.kernel.org/r/20230525124504.633469722@linutronix.deSigned-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmalloc.c

@@ -2086,39 +2086,54 @@ static void free_vmap_block(struct vmap_block *vb)
 	kfree_rcu(vb, rcu_head);
 }
 
+static bool purge_fragmented_block(struct vmap_block *vb,
+		struct vmap_block_queue *vbq, struct list_head *purge_list)
+{
+	if (vb->free + vb->dirty != VMAP_BBMAP_BITS ||
+	    vb->dirty == VMAP_BBMAP_BITS)
+		return false;
+
+	/* prevent further allocs after releasing lock */
+	vb->free = 0;
+	/* prevent purging it again */
+	vb->dirty = VMAP_BBMAP_BITS;
+	vb->dirty_min = 0;
+	vb->dirty_max = VMAP_BBMAP_BITS;
+	spin_lock(&vbq->lock);
+	list_del_rcu(&vb->free_list);
+	spin_unlock(&vbq->lock);
+	list_add_tail(&vb->purge, purge_list);
+	return true;
+}
+
+static void free_purged_blocks(struct list_head *purge_list)
+{
+	struct vmap_block *vb, *n_vb;
+
+	list_for_each_entry_safe(vb, n_vb, purge_list, purge) {
+		list_del(&vb->purge);
+		free_vmap_block(vb);
+	}
+}
+
 static void purge_fragmented_blocks(int cpu)
 {
 	LIST_HEAD(purge);
 	struct vmap_block *vb;
-	struct vmap_block *n_vb;
 	struct vmap_block_queue *vbq = &per_cpu(vmap_block_queue, cpu);
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(vb, &vbq->free, free_list) {
-
-		if (!(vb->free + vb->dirty == VMAP_BBMAP_BITS && vb->dirty != VMAP_BBMAP_BITS))
+		if (vb->free + vb->dirty != VMAP_BBMAP_BITS ||
+		    vb->dirty == VMAP_BBMAP_BITS)
 			continue;
 
 		spin_lock(&vb->lock);
-		if (vb->free + vb->dirty == VMAP_BBMAP_BITS && vb->dirty != VMAP_BBMAP_BITS) {
-			vb->free = 0; /* prevent further allocs after releasing lock */
-			vb->dirty = VMAP_BBMAP_BITS; /* prevent purging it again */
-			vb->dirty_min = 0;
-			vb->dirty_max = VMAP_BBMAP_BITS;
-			spin_lock(&vbq->lock);
-			list_del_rcu(&vb->free_list);
-			spin_unlock(&vbq->lock);
-			spin_unlock(&vb->lock);
-			list_add_tail(&vb->purge, &purge);
-		} else
-			spin_unlock(&vb->lock);
+		purge_fragmented_block(vb, vbq, &purge);
+		spin_unlock(&vb->lock);
 	}
 	rcu_read_unlock();
-
-	list_for_each_entry_safe(vb, n_vb, &purge, purge) {
-		list_del(&vb->purge);
-		free_vmap_block(vb);
-	}
+	free_purged_blocks(&purge);
 }
 
 static void purge_fragmented_blocks_allcpus(void)
@@ -2226,12 +2241,13 @@ static void vb_free(unsigned long addr, unsigned long size)
 
 static void _vm_unmap_aliases(unsigned long start, unsigned long end, int flush)
 {
+	LIST_HEAD(purge_list);
 	int cpu;
 
 	if (unlikely(!vmap_initialized))
 		return;
 
-	might_sleep();
+	mutex_lock(&vmap_purge_lock);
 
 	for_each_possible_cpu(cpu) {
 		struct vmap_block_queue *vbq = &per_cpu(vmap_block_queue, cpu);
@@ -2241,7 +2257,14 @@ static void _vm_unmap_aliases(unsigned long start, unsigned long end, int flush)
 		rcu_read_lock();
 		xa_for_each(&vbq->vmap_blocks, idx, vb) {
 			spin_lock(&vb->lock);
-			if (vb->dirty && vb->dirty != VMAP_BBMAP_BITS) {
+
+			/*
+			 * Try to purge a fragmented block first. If it's
+			 * not purgeable, check whether there is dirty
+			 * space to be flushed.
+			 */
+			if (!purge_fragmented_block(vb, vbq, &purge_list) &&
+			    vb->dirty && vb->dirty != VMAP_BBMAP_BITS) {
 				unsigned long va_start = vb->va->va_start;
 				unsigned long s, e;
 
@@ -2257,9 +2280,8 @@ static void _vm_unmap_aliases(unsigned long start, unsigned long end, int flush)
 		}
 		rcu_read_unlock();
 	}
+	free_purged_blocks(&purge_list);
 
-	mutex_lock(&vmap_purge_lock);
-	purge_fragmented_blocks_allcpus();
 	if (!__purge_vmap_area_lazy(start, end) && flush)
 		flush_tlb_kernel_range(start, end);
 	mutex_unlock(&vmap_purge_lock);