mm/vmalloc: dont purge usable blocks unnecessarily

Purging fragmented blocks is done unconditionally in several contexts:

  1) From drain_vmap_area_work(), when the number of lazy to be freed
     vmap_areas reached the threshold

  2) Reclaiming vmalloc address space from pcpu_get_vm_areas()

  3) _vm_unmap_aliases()

#1 There is no reason to zap fragmented vmap blocks unconditionally, simply
   because reclaiming all lazy areas drains at least

      32MB * fls(num_online_cpus())

   per invocation which is plenty.

#2 Reclaiming when running out of space or due to memory pressure makes a
   lot of sense

#3 _unmap_aliases() requires to touch everything because the caller has no
   clue which vmap_area used a particular page last and the vmap_area lost
   that information too.

   Except for the vfree + VM_FLUSH_RESET_PERMS case, which removes the
   vmap area first and then cares about the flush. That in turn requires
   a full walk of _all_ vmap areas including the one which was just
   added to the purge list.

   But as this has to be flushed anyway this is an opportunity to combine
   outstanding TLB flushes and do the housekeeping of purging freed areas,
   but like #1 there is no real good reason to zap usable vmap blocks
   unconditionally.

Add a @force_purge argument to the newly split out block purge function and
if not true only purge fragmented blocks which have less than 1/4 of their
capacity left.

Rename purge_vmap_area_lazy() to reclaim_and_purge_vmap_areas() to make it
clear what the function does.

[lstoakes@gmail.com: correct VMAP_PURGE_THRESHOLD check]
  Link: https://lkml.kernel.org/r/3e92ef61-b910-4576-88e7-cf43211fd4e7@lucifer.local
Link: https://lkml.kernel.org/r/20230525124504.864005691@linutronix.deSigned-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
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

@@ -791,7 +791,7 @@ get_subtree_max_size(struct rb_node *node)
 RB_DECLARE_CALLBACKS_MAX(static, free_vmap_area_rb_augment_cb,
 	struct vmap_area, rb_node, unsigned long, subtree_max_size, va_size)
 
-static void purge_vmap_area_lazy(void);
+static void reclaim_and_purge_vmap_areas(void);
 static BLOCKING_NOTIFIER_HEAD(vmap_notify_list);
 static void drain_vmap_area_work(struct work_struct *work);
 static DECLARE_WORK(drain_vmap_work, drain_vmap_area_work);
@@ -1649,7 +1649,7 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
 
 overflow:
 	if (!purged) {
-		purge_vmap_area_lazy();
+		reclaim_and_purge_vmap_areas();
 		purged = 1;
 		goto retry;
 	}
@@ -1785,9 +1785,10 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
 }
 
 /*
- * Kick off a purge of the outstanding lazy areas.
+ * Reclaim vmap areas by purging fragmented blocks and purge_vmap_area_list.
  */
-static void purge_vmap_area_lazy(void)
+static void reclaim_and_purge_vmap_areas(void)
+
 {
 	mutex_lock(&vmap_purge_lock);
 	purge_fragmented_blocks_allcpus();
@@ -1908,6 +1909,12 @@ static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
 
 #define VMAP_BLOCK_SIZE		(VMAP_BBMAP_BITS * PAGE_SIZE)
 
+/*
+ * Purge threshold to prevent overeager purging of fragmented blocks for
+ * regular operations: Purge if vb->free is less than 1/4 of the capacity.
+ */
+#define VMAP_PURGE_THRESHOLD	(VMAP_BBMAP_BITS / 4)
+
 #define VMAP_RAM		0x1 /* indicates vm_map_ram area*/
 #define VMAP_BLOCK		0x2 /* mark out the vmap_block sub-type*/
 #define VMAP_FLAGS_MASK		0x3
@@ -2087,12 +2094,17 @@ static void free_vmap_block(struct vmap_block *vb)
 }
 
 static bool purge_fragmented_block(struct vmap_block *vb,
-		struct vmap_block_queue *vbq, struct list_head *purge_list)
+		struct vmap_block_queue *vbq, struct list_head *purge_list,
+		bool force_purge)
 {
 	if (vb->free + vb->dirty != VMAP_BBMAP_BITS ||
 	    vb->dirty == VMAP_BBMAP_BITS)
 		return false;
 
+	/* Don't overeagerly purge usable blocks unless requested */
+	if (!(force_purge || vb->free < VMAP_PURGE_THRESHOLD))
+		return false;
+
 	/* prevent further allocs after releasing lock */
 	WRITE_ONCE(vb->free, 0);
 	/* prevent purging it again */
@@ -2132,7 +2144,7 @@ static void purge_fragmented_blocks(int cpu)
 			continue;
 
 		spin_lock(&vb->lock);
-		purge_fragmented_block(vb, vbq, &purge);
+		purge_fragmented_block(vb, vbq, &purge, true);
 		spin_unlock(&vb->lock);
 	}
 	rcu_read_unlock();
@@ -2269,7 +2281,7 @@ static void _vm_unmap_aliases(unsigned long start, unsigned long end, int flush)
 			 * not purgeable, check whether there is dirty
 			 * space to be flushed.
 			 */
-			if (!purge_fragmented_block(vb, vbq, &purge_list) &&
+			if (!purge_fragmented_block(vb, vbq, &purge_list, false) &&
 			    vb->dirty_max && vb->dirty != VMAP_BBMAP_BITS) {
 				unsigned long va_start = vb->va->va_start;
 				unsigned long s, e;
@@ -4175,7 +4187,7 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
 overflow:
 	spin_unlock(&free_vmap_area_lock);
 	if (!purged) {
-		purge_vmap_area_lazy();
+		reclaim_and_purge_vmap_areas();
 		purged = true;
 
 		/* Before "retry", check if we recover. */