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

[Miaohe Lin]

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()?

> 
>>> +}
>>> +
>>>  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
  ...

cpu2
get_swap_device()
  /* ignored  smp_rmb() */
  percpu_ref_tryget_live
  ...

There is indeed missing smp_rmb() in percpu_ref_tryget_live. So I think the above
scene possible and we should fix this.

>>>  	spin_lock(&swap_lock);
>>>  	spin_lock(&p->lock);
>>>  	_enable_swap_info(p);
>>> @@ -2621,11 +2631,13 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>>>  	p->flags &= ~SWP_VALID;		/* mark swap device as invalid */
>>>  	spin_unlock(&p->lock);
>>>  	spin_unlock(&swap_lock);
>>> +
>>> +	percpu_ref_kill(&p->users);
>>>  	/*
>>>  	 * wait for swap operations protected by get/put_swap_device()
>>>  	 * to complete
>>>  	 */
>>> -	synchronize_rcu();
>>> +	wait_for_completion(&p->comp);
>>
>> Better to move percpu_ref_kill() after the comments.  And maybe revise
>> the comments.
> 
> After reading the original commit description as above, I found that we
> need synchronize_rcu() here to protect the accessing to the swap cache
> data structure.  Because there's call_rcu() during percpu_ref_kill(), it
> appears OK to keep the synchronize_rcu() here.  And we need to revise
> the comments to make it clear what is protected by which operation.
> 

Per my understanding, percpu_ref->data->release is called only after the refcnt
reaches 0, including a full grace period has elapsed or refcnt won't be 0.
wait_for_completion() is used for waiting the last refcnt being released. So
synchronize_rcu() is not necessary here?

> Best Regards,

Many thanks for precious comments! :)

> Huang, Ying
> 
> [snip]
> .
>