Re: [RFC PATCH] mm/vmalloc: fix vbq->free breakage

[Hailong Liu]

On 5/31/2024 4:03 AM, Andrew Morton wrote:
> On Thu, 30 May 2024 17:31:08 +0800 <hailong.liu@xxxxxxxx> wrote:
> 
>> From: "hailong.liu" <hailong.liu@xxxxxxxx>
>>
>> The function xa_for_each() in _vm_unmap_aliases() loops through all
>> vbs. However, since commit 062eacf57ad9 ("mm: vmalloc: remove a global
>> vmap_blocks xarray") the vb from xarray may not be on the corresponding
>> CPU vmap_block_queue. Consequently, purge_fragmented_block() might
>> use the wrong vbq->lock to protect the free list, leading to vbq->free
>> breakage.
> 
> What are the userspace-visible runtime effects of this change?
> 
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index d12a17fc0c17..869e7788a7d5 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -2269,10 +2269,9 @@ static void _vm_unmap_aliases(unsigned long start, unsigned long end, int flush)
>>  	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();
>> -		xa_for_each(&vbq->vmap_blocks, idx, vb) {
>> +		list_for_each_entry_rcu(vb, &vbq->free, free_list) {
>>  			spin_lock(&vb->lock);
>>
>>  			/*
>> ---
>> https://lore.kernel.org/all/20240530025144.1570865-1-zhaoyang.huang@xxxxxxxxxx/
>> BTW, zhangyang also encounter the same issue, maybe revert commit not a
>> better solution. we need a map to get vbq from vb.
> 
> That patch didn't describe the runtime effects either.
> 
> Folks, please always do this.  So that others can decide whether their
> kernel needs the fix, and so that others can decide whether this fix
> might address an issue which their users are reporting.

Thanks for suggestion. I send v2 in 
https://lore.kernel.org/lkml/20240531024820.5507-1-hailong.liu@xxxxxxxx/T/#u

I promise to make my future commit messages comprehensive, in order to help others.

Brs,
Hailong.