[patch 4/6] mm/vmalloc: Check free space in vmap_block lockless

Thomas Gleixner
2023-05-23T14:05:32Z

vb_alloc() unconditionally locks a vmap_block on the free list to check the
free space.

This can be done locklessly because vmap_block::free never increases, it's
only decreased on allocations.

Check the free space lockless and only if that succeeds, recheck under the
lock.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
mm/vmalloc.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2166,6 +2166,9 @@ static void *vb_alloc(unsigned long size
list_for_each_entry_rcu(vb, &vbq->free, free_list) {
unsigned long pages_off;

+ if (READ_ONCE(vb->free) < (1UL << order))
+ continue;
+
spin_lock(&vb->lock);
if (vb->free < (1UL << order)) {
spin_unlock(&vb->lock);
@@ -2174,7 +2177,7 @@ static void *vb_alloc(unsigned long size

pages_off = VMAP_BBMAP_BITS - vb->free;
vaddr = vmap_block_vaddr(vb->va->va_start, pages_off);
- vb->free -= 1UL << order;
+ WRITE_ONCE(vb->free, vb->free - (1UL << order));
bitmap_set(vb->used_map, pages_off, (1UL << order));
if (vb->free == 0) {
spin_lock(&vbq->lock);