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

[Miaohe Lin]

On 2021/4/16 14:25, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
> 
>> On 2021/4/15 22:31, Dennis Zhou wrote:
>>> On Thu, Apr 15, 2021 at 01:24:31PM +0800, Huang, Ying wrote:
>>>> Dennis Zhou <dennis@xxxxxxxxxx> writes:
>>>>
>>>>> On Wed, Apr 14, 2021 at 01:44:58PM +0800, Huang, Ying wrote:
>>>>>> Dennis Zhou <dennis@xxxxxxxxxx> writes:
>>>>>>
>>>>>>> On Wed, Apr 14, 2021 at 11:59:03AM +0800, Huang, Ying wrote:
>>>>>>>> Dennis Zhou <dennis@xxxxxxxxxx> writes:
>>>>>>>>
>>>>>>>>> Hello,
>>>>>>>>>
>>>>>>>>> On Wed, Apr 14, 2021 at 10:06:48AM +0800, Huang, Ying wrote:
>>>>>>>>>> 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.
>>>>>>>>>
>>>>>>>>> I'm just catching up and following along a little bit. I apologize I
>>>>>>>>> haven't read the swap code, but my understanding is you are trying to
>>>>>>>>> narrow a race condition with swapoff. That makes sense to me. I'm not
>>>>>>>>> sure I follow the need to race with reinitializing the ref though? Is it
>>>>>>>>> not possible to wait out the dying swap info and then create a new one
>>>>>>>>> rather than push acquire semantics?
>>>>>>>>
>>>>>>>> We want to check whether the swap entry is valid (that is, the swap
>>>>>>>> device isn't swapped off now), prevent it from swapping off, then access
>>>>>>>> the swap_info_struct data structure.  When accessing swap_info_struct,
>>>>>>>> we want to guarantee the ordering, so that we will not reference
>>>>>>>> uninitialized fields of swap_info_struct.
>>>>>>>>
>>>>>>>
>>>>>>> So in the normal context of percpu_ref, once someone can access it, the
>>>>>>> elements that it is protecting are expected to be initialized.
>>>>>>
>>>>>> If we can make sure that all elements being initialized fully, why not
>>>>>> just use percpu_ref_get() instead of percpu_ref_tryget*()?
>>>>>>
>>>>>
>>>>> Generally, the lookup is protected with rcu and then
>>>>> percpu_ref_tryget*() is used to obtain a reference. percpu_ref_get() is
>>>>> only good if you already have a ref as it increments regardless of being
>>>>> 0.
>>>>>
>>>>> What I mean is if you can get a ref, that means the object hasn't been
>>>>> destroyed. This differs from the semantics you are looking for which I
>>>>> understand to be: I have long lived pointers to objects. The object may
>>>>> die, but I may resurrect it and I want the old pointers to still be
>>>>> valid.
>>>>>
>>>>> When is it possible for someone to have a pointer to the swap device and
>>>>> the refcount goes to 0? It might be better to avoid this situation than
>>>>> add acquire semantics.
>>>>>
>>>>>>> In the basic case for swap off, I'm seeing the goal as to prevent
>>>>>>> destruction until anyone currently accessing swap is done. In this
>>>>>>> case wouldn't we always be protecting a live struct?
>>>>>>>
>>>>>>> I'm maybe not understanding what conditions you're trying to revive the
>>>>>>> percpu_ref?
>>>>>>
>>>>>> A swap entry likes an indirect pointer to a swap device.  We may hold a
>>>>>> swap entry for long time, so that the swap device is swapoff/swapon.
>>>>>> Then we need to make sure the swap device are fully initialized before
>>>>>> accessing the swap device via the swap entry.
>>>>>>
>>>>>
>>>>> So if I have some number of outstanding references, and then
>>>>> percpu_ref_kill() is called, then only those that have the pointer will
>>>>> be able to use the swap device as those references are still good. Prior
>>>>> to calling percpu_ref_kill(), call_rcu() needs to be called on lookup
>>>>> data structure.
>>>>>
>>>>> My personal understanding of tryget() vs tryget_live() is that it
>>>>> provides a 2 phase clean up and bounds the ability for new users to come
>>>>> in (cgroup destruction is a primary user). As tryget() might inevitably
>>>>> let a cgroup live long past its removal, tryget_live() will say oh
>>>>> you're in the process of dying do something else.
>>>>
>>>> OK.  I think that I understand your typical use case now.  The resource
>>>> producer code may look like,
>>>>
>>>>   obj = kmalloc();
>>>>   /* Initialize obj fields */
>>>>   percpu_ref_init(&obj->ref);
>>>>   rcu_assign_pointer(global_p, obj);
>>>>
>>>> The resource reclaimer looks like,
>>>>
>>>>   p = global_p;
>>>>   global_p = NULL;
>>>>   percpu_ref_kill(&p->ref);
>>>>   /* wait until percpu_ref_is_zero(&p->ref) */
>>>>   /* free resources pointed by obj fields */
>>>>   kfree(p);
>>>>
>>>> The resource producer looks like,
>>>>
>>>>   rcu_read_lock();
>>>>   p = rcu_dereference(global_p);
>>>>   if (!p || !percpu_ref_tryget_live(&p->ref)) {
>>>>           /* Invalid pointer, go out */
>>>>   }
>>>>   rcu_read_unlock();
>>>>   /* use p */
>>>>   percpu_ref_put(&p->ref);
>>>>
>>>> For this use case, it's not necessary to make percpu_ref_tryget_live()
>>>> ACQUIRE operation.  Because refcount doesn't act as a flag to indicate
>>>> whether the object has been fully initialized, global_p does.  And
>>>> the data dependency guaranteed the required ordering.
>>>>
>>>
>>> Yes this is spot on.
>>>
>>>> The use case of swap is different.  Where global_p always points to
>>>> the obj (never freed) even if the resources pointed by obj fields has
>>>> been freed.  And we want to use refcount as a flag to indicate whether
>>>> the object is fully initialized.  This is hard to be changed, because
>>>> the global_p is used to identify the stalled pointer from the totally
>>>> invalid pointer.
>>>>
>>>
>>> Apologies ahead of time for this possibly dumb question. Is it possible
>>> to have swapon swap out the global_p with
>>> old_obj = rcu_access_pointer(global_p);
>>> rcu_assign_pointer(global_p, obj);
>>> kfree_rcu(remove_old_obj) or call_rcu();
>>>
>>> Then the obj pointed to by global_p would always be valid, but only
>>> would be alive again if it got the new pointer?
>>
>> Many thanks for both of you! Looks like a nice solution! Will try to do it in v2.
>> Thanks again! :)
> 
> Think about this again.  This means that we need to free the old
> swap_info_struct at some time.  So something like RCU is needed to
> enclose the accessor.  But some accessor doesn't follow this, and it
> appears overkill to change all these accessors.  So I think at least as
> the first step, smp_rmb() appears more appropriate.
> 

Agree. Thanks！

> Best Regards,
> Huang, Ying
> 
>>>
>>>> If all other users follow the typical use case above, we may find some
>>>> other way to resolve the problem inside swap code, such as adding
>>>> smp_rmb() after percpu_ref_tryget_live().
>>>>
>>>
>>> I would prefer it.
>>>
>>>> Best Regards,
>>>> Huang, Ying
>>>
>>> Thanks,
>>> Dennis
>>>
>>> .
>>>
> 
> .
>