Re: [PATCHv3] mm: fix incorrect vbq reference in purge_fragmented_block

[Hailong Liu]

On 5/31/2024 6:44 PM, Hillf Danton wrote:
> On Fri, 31. May 10:04, Uladzislau Rezki wrote:
>> On Fri, May 31, 2024 at 11:05:20AM +0800, zhaoyang.huang wrote:
>>> From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
>>>
>>> vmalloc area runs out in our ARM64 system during an erofs test as
>>> vm_map_ram failed[1]. By following the debug log, we find that
>>> vm_map_ram()->vb_alloc() will allocate new vb->va which corresponding
>>> to 4MB vmalloc area as list_for_each_entry_rcu returns immediately
>>> when vbq->free->next points to vbq->free. That is to say, 65536 times
>>> of page fault after the list's broken will run out of the whole
>>> vmalloc area. This should be introduced by one vbq->free->next point to
>>> vbq->free which makes list_for_each_entry_rcu can not iterate the list
>>> and find the BUG.
>>>
>>> [1]
>>> PID: 1        TASK: ffffff80802b4e00  CPU: 6    COMMAND: "init"
>>>  #0 [ffffffc08006afe0] __switch_to at ffffffc08111d5cc
>>>  #1 [ffffffc08006b040] __schedule at ffffffc08111dde0
>>>  #2 [ffffffc08006b0a0] schedule at ffffffc08111e294
>>>  #3 [ffffffc08006b0d0] schedule_preempt_disabled at ffffffc08111e3f0
>>>  #4 [ffffffc08006b140] __mutex_lock at ffffffc08112068c
>>>  #5 [ffffffc08006b180] __mutex_lock_slowpath at ffffffc08111f8f8
>>>  #6 [ffffffc08006b1a0] mutex_lock at ffffffc08111f834
>>>  #7 [ffffffc08006b1d0] reclaim_and_purge_vmap_areas at ffffffc0803ebc3c
>>>  #8 [ffffffc08006b290] alloc_vmap_area at ffffffc0803e83fc
>>>  #9 [ffffffc08006b300] vm_map_ram at ffffffc0803e78c0
>>>
>>> Fixes: fc1e0d980037 ("mm/vmalloc: prevent stale TLBs in fully utilized blocks")
>>>
>>> Suggested-by: Hailong.Liu <hailong.liu@xxxxxxxx>
>>> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
>>>
>> Is a problem related to run out of vmalloc space _only_ or it is a problem
>> with broken list? From the commit message it is hard to follow the reason.
>>
>> Could you please post a full trace or panic?
> 
> What they proposed looks correct IIUC
> 
> --- l/mm/vmalloc.c
> +++ v/mm/vmalloc.c
> @@ -2067,7 +2067,7 @@ static void *new_vmap_block(unsigned int
>  		return ERR_PTR(err);
>  	}
>  
> -	vbq = raw_cpu_ptr(&vmap_block_queue);
> +	vbq = container_of(xa, struct vmap_block_queue, vmap_blocks);
IMO, this violates the percpu principle, why not use vmap_block_queues[NR_CPUS]?
>  	spin_lock(&vbq->lock);
>  	list_add_tail_rcu(&vb->free_list, &vbq->free);
>  	spin_unlock(&vbq->lock);