Re: [PATCH 1/5] mm/swapfile: add percpu_ref support for swap

[Huang, Ying]

Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:

> On 2021/4/14 9:17, Huang, Ying wrote:
>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>> 
>>> On 2021/4/12 15:24, Huang, Ying wrote:
>>>> "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
>>>>
>>>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>>>>>
>>>>>> We will use percpu-refcount to serialize against concurrent swapoff. This
>>>>>> patch adds the percpu_ref support for later fixup.
>>>>>>
>>>>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>>>>> ---
>>>>>>  include/linux/swap.h |  2 ++
>>>>>>  mm/swapfile.c        | 25 ++++++++++++++++++++++---
>>>>>>  2 files changed, 24 insertions(+), 3 deletions(-)
>>>>>>
>>>>>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>>>>>> index 144727041e78..849ba5265c11 100644
>>>>>> --- a/include/linux/swap.h
>>>>>> +++ b/include/linux/swap.h
>>>>>> @@ -240,6 +240,7 @@ struct swap_cluster_list {
>>>>>>   * The in-memory structure used to track swap areas.
>>>>>>   */
>>>>>>  struct swap_info_struct {
>>>>>> +	struct percpu_ref users;	/* serialization against concurrent swapoff */
>>>>>>  	unsigned long	flags;		/* SWP_USED etc: see above */
>>>>>>  	signed short	prio;		/* swap priority of this type */
>>>>>>  	struct plist_node list;		/* entry in swap_active_head */
>>>>>> @@ -260,6 +261,7 @@ struct swap_info_struct {
>>>>>>  	struct block_device *bdev;	/* swap device or bdev of swap file */
>>>>>>  	struct file *swap_file;		/* seldom referenced */
>>>>>>  	unsigned int old_block_size;	/* seldom referenced */
>>>>>> +	struct completion comp;		/* seldom referenced */
>>>>>>  #ifdef CONFIG_FRONTSWAP
>>>>>>  	unsigned long *frontswap_map;	/* frontswap in-use, one bit per page */
>>>>>>  	atomic_t frontswap_pages;	/* frontswap pages in-use counter */
>>>>>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>>>>>> index 149e77454e3c..724173cd7d0c 100644
>>>>>> --- a/mm/swapfile.c
>>>>>> +++ b/mm/swapfile.c
>>>>>> @@ -39,6 +39,7 @@
>>>>>>  #include <linux/export.h>
>>>>>>  #include <linux/swap_slots.h>
>>>>>>  #include <linux/sort.h>
>>>>>> +#include <linux/completion.h>
>>>>>>  
>>>>>>  #include <asm/tlbflush.h>
>>>>>>  #include <linux/swapops.h>
>>>>>> @@ -511,6 +512,15 @@ static void swap_discard_work(struct work_struct *work)
>>>>>>  	spin_unlock(&si->lock);
>>>>>>  }
>>>>>>  
>>>>>> +static void swap_users_ref_free(struct percpu_ref *ref)
>>>>>> +{
>>>>>> +	struct swap_info_struct *si;
>>>>>> +
>>>>>> +	si = container_of(ref, struct swap_info_struct, users);
>>>>>> +	complete(&si->comp);
>>>>>> +	percpu_ref_exit(&si->users);
>>>>>
>>>>> Because percpu_ref_exit() is used, we cannot use percpu_ref_tryget() in
>>>>> get_swap_device(), better to add comments there.
>>>>
>>>> I just noticed that the comments of percpu_ref_tryget_live() says,
>>>>
>>>>  * This function is safe to call as long as @ref is between init and exit.
>>>>
>>>> While we need to call get_swap_device() almost at any time, so it's
>>>> better to avoid to call percpu_ref_exit() at all.  This will waste some
>>>> memory, but we need to follow the API definition to avoid potential
>>>> issues in the long term.
>>>
>>> I have to admit that I'am not really familiar with percpu_ref. So I read the
>>> implementation code of the percpu_ref and found percpu_ref_tryget_live() could
>>> be called after exit now. But you're right we need to follow the API definition
>>> to avoid potential issues in the long term.
>>>
>>>>
>>>> And we need to call percpu_ref_init() before insert the swap_info_struct
>>>> into the swap_info[].
>>>
>>> If we remove the call to percpu_ref_exit(), we should not use percpu_ref_init()
>>> here because *percpu_ref->data is assumed to be NULL* in percpu_ref_init() while
>>> this is not the case as we do not call percpu_ref_exit(). Maybe percpu_ref_reinit()
>>> or percpu_ref_resurrect() will do the work.
>>>
>>> One more thing, how could I distinguish the killed percpu_ref from newly allocated one?
>>> It seems percpu_ref_is_dying is only safe to call when @ref is between init and exit.
>>> Maybe I could do this in alloc_swap_info()?
>> 
>> Yes.  In alloc_swap_info(), you can distinguish newly allocated and
>> reused swap_info_struct.
>> 
>>>>
>>>>>> +}
>>>>>> +
>>>>>>  static void alloc_cluster(struct swap_info_struct *si, unsigned long idx)
>>>>>>  {
>>>>>>  	struct swap_cluster_info *ci = si->cluster_info;
>>>>>> @@ -2500,7 +2510,7 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
>>>>>>  	 * Guarantee swap_map, cluster_info, etc. fields are valid
>>>>>>  	 * between get/put_swap_device() if SWP_VALID bit is set
>>>>>>  	 */
>>>>>> -	synchronize_rcu();
>>>>>> +	percpu_ref_reinit(&p->users);
>>>>>
>>>>> Although the effect is same, I think it's better to use
>>>>> percpu_ref_resurrect() here to improve code readability.
>>>>
>>>> Check the original commit description for commit eb085574a752 "mm, swap:
>>>> fix race between swapoff and some swap operations" and discussion email
>>>> thread as follows again,
>>>>
>>>> https://lore.kernel.org/linux-mm/20171219053650.GB7829@xxxxxxxxxxxxxxxxxx/
>>>>
>>>> I found that the synchronize_rcu() here is to avoid to call smp_rmb() or
>>>> smp_load_acquire() in get_swap_device().  Now we will use
>>>> percpu_ref_tryget_live() in get_swap_device(), so we will need to add
>>>> the necessary memory barrier, or make sure percpu_ref_tryget_live() has
>>>> ACQUIRE semantics.  Per my understanding, we need to change
>>>> percpu_ref_tryget_live() for that.
>>>>
>>>
>>> Do you mean the below scene is possible?
>>>
>>> cpu1
>>> swapon()
>>>   ...
>>>   percpu_ref_init
>>>   ...
>>>   setup_swap_info
>>>   /* smp_store_release() is inside percpu_ref_reinit */
>>>   percpu_ref_reinit
>> 
>> spin_unlock() has RELEASE semantics already.
>> 
>>>   ...
>>>
>>> cpu2
>>> get_swap_device()
>>>   /* ignored  smp_rmb() */
>>>   percpu_ref_tryget_live
>> 
>> Some kind of ACQUIRE is required here to guarantee the refcount is
>> checked before fetching the other fields of swap_info_struct.  I have
>> sent out a RFC patch to mailing list to discuss this.
>
> Many thanks.
> But We may still need to add a smp_rmb() in get_swap_device() in case
> we can't add ACQUIRE for refcount.

Yes.

Best Regards,
Huang, Ying