Re: [patch 1/6] mm/vmalloc: Prevent stale TLBs in fully utilized blocks

Lorenzo Stoakes
2023-05-23T19:18:17Z

On Tue, May 23, 2023 at 04:02:11PM +0200, Thomas Gleixner wrote:
> _vm_unmap_aliases() is used to ensure that no unflushed TLB entries for a
> page are left in the system. This is required due to the lazy TLB flush
> mechanism in vmalloc.
>
> This is tried to achieve by walking the per CPU free lists, but those do
> not contain fully utilized vmap blocks because they are removed from the
> free list once the blocks free space became zero.

Oh Lord.

>
> So the per CPU list iteration does not find the block and if the page was
> mapped via such a block and the TLB has not yet been flushed, the guarantee
> of _vm_unmap_aliases() that there are no stale TLBs after returning is
> broken:
>
> x = vb_alloc() // Removes vmap_block from free list because vb->free became 0
> vb_free(x) // Unmaps page and marks in dirty_min/max range
>
> // Page is reused
> vm_unmap_aliases() // Can't find vmap block with the dirty space -> FAIL
>
> So instead of walking the per CPU free lists, walk the per CPU xarrays
> which hold pointers to _all_ active blocks in the system including those
> removed from the free lists.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> mm/vmalloc.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2236,9 +2236,10 @@ static void _vm_unmap_aliases(unsigned l
> for_each_possible_cpu(cpu) {
> struct vmap_block_queue *vbq = &per_cpu(vmap_block_queue, cpu);
> struct vmap_block *vb;
> + unsigned long idx;
>
> rcu_read_lock();
> - list_for_each_entry_rcu(vb, &vbq->free, free_list) {
> + xa_for_each(&vbq->vmap_blocks, idx, vb) {
> spin_lock(&vb->lock);
> if (vb->dirty && vb->dirty != VMAP_BBMAP_BITS) {
> unsigned long va_start = vb->va->va_start;
>
>

LGTM.

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