Re: [RFC PATCH v2-next] mm: memory-failure: use rcu lock instead of tasklist_lock when collect_procs()

[Kefeng Wang]

On 2023/8/18 16:17, Tong Tiangen wrote:
> We found a softlock issue in our test, analyzed the logs, and found that
> the relevant CPU call trace as follows:
>
> CPU0:
>    _do_fork
>      -> copy_process()
>        -> write_lock_irq(&tasklist_lock)  //Disable irq,waiting for
>        					 //tasklist_lock
>
> CPU1:
>    wp_page_copy()
>      ->pte_offset_map_lock()
>        -> spin_lock(&page->ptl);        //Hold page->ptl
>      -> ptep_clear_flush()
>        -> flush_tlb_others() ...
>          -> smp_call_function_many()
>            -> arch_send_call_function_ipi_mask()
>              -> csd_lock_wait()         //Waiting for other CPUs respond
> 	                               //IPI
>
> CPU2:
>    collect_procs_anon()
>      -> read_lock(&tasklist_lock)       //Hold tasklist_lock
>        ->for_each_process(tsk)
>          -> page_mapped_in_vma()
>            -> page_vma_mapped_walk()
> 	    -> map_pte()
>                ->spin_lock(&page->ptl)  //Waiting for page->ptl
>
> We can see that CPU1 waiting for CPU0 respond IPI，CPU0 waiting for CPU2
> unlock tasklist_lock, CPU2 waiting for CPU1 unlock page->ptl. As a result,
> softlockup is triggered.
>
> For collect_procs_anon(), we will not modify the tasklist, but only perform
> read traversal. Therefore, we can use rcu lock instead of spin lock
> tasklist_lock, from this, we can break the softlock chain above.
>
> The same logic can also be applied to:
>   - collect_procs_file()
>   - collect_procs_fsdax()
>   - collect_procs_ksm()
>   - find_early_kill_thread()
>
> Signed-off-by: Tong Tiangen <tongtiangen@xxxxxxxxxx>
> ---
> v2:
>   - 1. Modify the title description.
>   - 2. Optimize the implementation of find_early_kill_thread() without
>        functional changes.
> ---

Those changes are fine to me， please fix the comment mentioned by  Noaoya.

>   mm/ksm.c            |  4 ++--
>   mm/memory-failure.c | 33 +++++++++++++++++++--------------
>   2 files changed, 21 insertions(+), 16 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 6b7b8928fb96..dcbc0c7f68e7 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2919,7 +2919,7 @@ void collect_procs_ksm(struct page *page, struct list_head *to_kill,
>   		struct anon_vma *av = rmap_item->anon_vma;
>   
>   		anon_vma_lock_read(av);
> -		read_lock(&tasklist_lock);
> +		rcu_read_lock();
>   		for_each_process(tsk) {
>   			struct anon_vma_chain *vmac;
>   			unsigned long addr;
> @@ -2938,7 +2938,7 @@ void collect_procs_ksm(struct page *page, struct list_head *to_kill,
>   				}
>   			}
>   		}
> -		read_unlock(&tasklist_lock);
> +		rcu_read_unlock();
>   		anon_vma_unlock_read(av);
>   	}
>   }
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 7b01fffe7a79..4f3081f47798 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -546,24 +546,29 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
>    * Find a dedicated thread which is supposed to handle SIGBUS(BUS_MCEERR_AO)
>    * on behalf of the thread group. Return task_struct of the (first found)
>    * dedicated thread if found, and return NULL otherwise.
> - *
> - * We already hold read_lock(&tasklist_lock) in the caller, so we don't
> - * have to call rcu_read_lock/unlock() in this function.
>    */
>   static struct task_struct *find_early_kill_thread(struct task_struct *tsk)
>   {
>   	struct task_struct *t;
> +	bool found = false;
>   
> +	rcu_read_lock();
>   	for_each_thread(tsk, t) {
>   		if (t->flags & PF_MCE_PROCESS) {
> -			if (t->flags & PF_MCE_EARLY)
> -				return t;
> +			if (t->flags & PF_MCE_EARLY) {
> +				found = true;
> +				break;
> +			}
>   		} else {
> -			if (sysctl_memory_failure_early_kill)
> -				return t;
> +			if (sysctl_memory_failure_early_kill) {
> +				found = true;
> +				break;
> +			}
>   		}
>   	}
> -	return NULL;
> +	rcu_read_unlock();
> +
> +	return found ? t : NULL;
>   }
>   
>   /*
> @@ -609,7 +614,7 @@ static void collect_procs_anon(struct page *page, struct list_head *to_kill,
>   		return;
>   
>   	pgoff = page_to_pgoff(page);
> -	read_lock(&tasklist_lock);
> +	rcu_read_lock();
>   	for_each_process(tsk) {
>   		struct anon_vma_chain *vmac;
>   		struct task_struct *t = task_early_kill(tsk, force_early);
> @@ -626,7 +631,7 @@ static void collect_procs_anon(struct page *page, struct list_head *to_kill,
>   			add_to_kill_anon_file(t, page, vma, to_kill);
>   		}
>   	}
> -	read_unlock(&tasklist_lock);
> +	rcu_read_unlock();
>   	anon_vma_unlock_read(av);
>   }
>   
> @@ -642,7 +647,7 @@ static void collect_procs_file(struct page *page, struct list_head *to_kill,
>   	pgoff_t pgoff;
>   
>   	i_mmap_lock_read(mapping);
> -	read_lock(&tasklist_lock);
> +	rcu_read_lock();
>   	pgoff = page_to_pgoff(page);
>   	for_each_process(tsk) {
>   		struct task_struct *t = task_early_kill(tsk, force_early);
> @@ -662,7 +667,7 @@ static void collect_procs_file(struct page *page, struct list_head *to_kill,
>   				add_to_kill_anon_file(t, page, vma, to_kill);
>   		}
>   	}
> -	read_unlock(&tasklist_lock);
> +	rcu_read_unlock();
>   	i_mmap_unlock_read(mapping);
>   }
>   
> @@ -685,7 +690,7 @@ static void collect_procs_fsdax(struct page *page,
>   	struct task_struct *tsk;
>   
>   	i_mmap_lock_read(mapping);
> -	read_lock(&tasklist_lock);
> +	rcu_read_lock();
>   	for_each_process(tsk) {
>   		struct task_struct *t = task_early_kill(tsk, true);
>   
> @@ -696,7 +701,7 @@ static void collect_procs_fsdax(struct page *page,
>   				add_to_kill_fsdax(t, page, vma, to_kill, pgoff);
>   		}
>   	}
> -	read_unlock(&tasklist_lock);
> +	rcu_read_unlock();
>   	i_mmap_unlock_read(mapping);
>   }
>   #endif /* CONFIG_FS_DAX */