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

Thomas Gleixner
2023-05-24T09:51:40Z

On Wed, May 24 2023 at 17:25, Baoquan He wrote:
> On 05/23/23 at 04:02pm, 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.
>
> The problem description is not accurate. This is tried to achieve for
> va associated with vmap_block by walking the per CPU free lists, those
> fully utilized vmap blocks can still be flushed in __purge_vmap_area_lazy()
> by calculating the [min:max] of purge_vmap_area_list, because va of
> vmap_blocks will be added to purge_vmap_area_list too via vb_free().

No. The fully utilized block cannot be purged when there are still
active mappings on it. Again:

X = vb_alloc()
...
Y = vb_alloc()
vb->free -= order;
if (!vb->vb_free)
list_del(vb->free_list);
...
vb_free(Y)
vb->dirty += order;
if (vb->dirty == VMAP_BBMAP_BITS) // Condition is _false_
free_block();

So because $X is not yet unmapped the block is neither on the free list
nor on purge_vmap_area_list.

Thanks,

tglx