Re: [V2 patch 4/6] mm/vmalloc: Check free space in vmap_block lockless

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Thu, May 25, 2023 at 02:57:07PM +0200, Thomas Gleixner wrote:
> 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>
> Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
>  mm/vmalloc.c |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2168,6 +2168,9 @@ 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);
> @@ -2176,7 +2179,7 @@ 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);
>
>

Looks good to me,

Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>