Re: [PATCH linux-next] mm: swap: get rid of deadloop in swapin readahead

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

On Mon, 21 Feb 2022 11:17:49 +0000 cgel.zte@gmail.com wrote:

> From: Guo Ziliang <guo.ziliang@zte.com.cn>
> 
> In our testing, a deadloop task was found. Through sysrq printing, same 
> stack was found every time, as follows:
> __swap_duplicate+0x58/0x1a0
> swapcache_prepare+0x24/0x30
> __read_swap_cache_async+0xac/0x220
> read_swap_cache_async+0x58/0xa0
> swapin_readahead+0x24c/0x628
> do_swap_page+0x374/0x8a0
> __handle_mm_fault+0x598/0xd60
> handle_mm_fault+0x114/0x200
> do_page_fault+0x148/0x4d0
> do_translation_fault+0xb0/0xd4
> do_mem_abort+0x50/0xb0
> 
> The reason for the deadloop is that swapcache_prepare() always returns
> EEXIST, indicating that SWAP_HAS_CACHE has not been cleared, so that
> it cannot jump out of the loop. We suspect that the task that clears
> the SWAP_HAS_CACHE flag never gets a chance to run. We try to lower
> the priority of the task stuck in a deadloop so that the task that
> clears the SWAP_HAS_CACHE flag will run. The results show that the
> system returns to normal after the priority is lowered.
> 
> In our testing, multiple real-time tasks are bound to the same core,
> and the task in the deadloop is the highest priority task of the
> core, so the deadloop task cannot be preempted.
> 
> Although cond_resched() is used by __read_swap_cache_async, it is an
> empty function in the preemptive system and cannot achieve the purpose
> of releasing the CPU. A high-priority task cannot release the CPU
> unless preempted by a higher-priority task. But when this task
> is already the highest priority task on this core, other tasks
> will not be able to be scheduled. So we think we should replace
> cond_resched() with schedule_timeout_uninterruptible(1),
> schedule_timeout_interruptible will call set_current_state
> first to set the task state, so the task will be removed
> from the running queue, so as to achieve the purpose of
> giving up the CPU and prevent it from running in kernel
> mode for too long.
> 
> ...
>
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -478,7 +478,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  		 * __read_swap_cache_async(), which has set SWAP_HAS_CACHE
>  		 * in swap_map, but not yet added its page to swap cache.
>  		 */
> -		cond_resched();
> +		schedule_timeout_uninterruptible(1);
>  	}
>  
>  	/*

Sigh.  I guess yes, we should do this, at least in a short-term,
backportable-to-stable way.

But busy-waiting while hoping that someone else will save us isn't an
attractive design.  Hugh, have you ever thought about something more
deterministic in there?

Thanks.