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

[Uladzislau Rezki]

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?

> ---
> v2: introduce cpu in vmap_block to record the right CPU number
> v3: use get_cpu/put_cpu to prevent schedule between core
> ---
> ---
>  mm/vmalloc.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 22aa63f4ef63..ecdb75d10949 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2458,6 +2458,7 @@ struct vmap_block {
>  	struct list_head free_list;
>  	struct rcu_head rcu_head;
>  	struct list_head purge;
> +	unsigned int cpu;
>  };
>  
>  /* Queue of free and dirty vmap blocks, for allocation and flushing purposes */
> @@ -2586,10 +2587,12 @@ static void *new_vmap_block(unsigned int order, gfp_t gfp_mask)
>  		return ERR_PTR(err);
>  	}
>  
> +	vb->cpu = get_cpu();
>  	vbq = raw_cpu_ptr(&vmap_block_queue);
>  	spin_lock(&vbq->lock);
>  	list_add_tail_rcu(&vb->free_list, &vbq->free);
>  	spin_unlock(&vbq->lock);
> +	put_cpu();
>  
Why do you need get_cpu() here? Can you go with raw_smp_processor_id()
and then access the per-cpu "vmap_block_queue"? get_cpu() disables
preemption and then a spin-lock is take within this critical section.
>From the first glance PREEMPT_RT is broken in this case.

I am on a vacation, responds can be with delays.

--
Uladzislau Rezki