Re: [PATCH v2] mm/swap: fix swap_info_struct race between swapoff and get_swap_pages()

[Andrew Morton <akpm@linux-foundation.org>]

On Tue,  4 Apr 2023 23:47:16 +0800 Rongwei Wang <rongwei.wang@linux.alibaba.com> wrote:

> The si->lock must be held when deleting the si from
> the available list.
>
> ...
>
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -679,6 +679,7 @@ static void __del_from_avail_list(struct swap_info_struct *p)
>  {
>  	int nid;
>  
> +	assert_spin_locked(&p->lock);
>  	for_each_node(nid)
>  		plist_del(&p->avail_lists[nid], &swap_avail_heads[nid]);
>  }
> @@ -2434,8 +2435,8 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  		spin_unlock(&swap_lock);
>  		goto out_dput;
>  	}
> -	del_from_avail_list(p);
>  	spin_lock(&p->lock);
> +	del_from_avail_list(p);
>  	if (p->prio < 0) {
>  		struct swap_info_struct *si = p;
>  		int nid;

So we have

swap_avail_lock
swap_info_struct.lock
swap_cluster_info.lock

Is the ranking of these three clearly documented somewhere?


Did you test this with lockdep fully enabled?


I'm thinking that Aaron's a2468cc9bfdff ("swap: choose swap device
according to numa node") is the appropriate Fixes: target - do you
agree?


These functions use identifier `p' for the swap_info_struct*, whereas
most other code uses the much more sensible `si'.  That's just rude. 
But we shouldn't change that within this fix.