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

[Naoya Horiguchi]

On Mon, Aug 21, 2023 at 10:25:34AM +0800, 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>

Acked-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>

> ---
> Changes since RFC[1]:
>  - 1. According to Naoya's suggestion, modify the tasklist_lock in the
>       comment about locking order in mm/filemap.c.
>  - 2. According to Kefeng's suggestion, optimize the implementation of
>       find_early_kill_thread() without functional changes.
>  - 3. Modify the title description.
> 
> [1] https://lore.kernel.org/lkml/20230815130154.1100779-1-tongtiangen@xxxxxxxxxx/