Date:   Fri, 16 Apr 2021 10:27:11 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Dennis Zhou <dennis@...nel.org>,
        "Huang, Ying" <ying.huang@...el.com>
CC:     <akpm@...ux-foundation.org>, <hannes@...xchg.org>,
        <mhocko@...e.com>, <iamjoonsoo.kim@....com>, <vbabka@...e.cz>,
        <alex.shi@...ux.alibaba.com>, <willy@...radead.org>,
        <minchan@...nel.org>, <richard.weiyang@...il.com>,
        <hughd@...gle.com>, <tim.c.chen@...ux.intel.com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 1/5] mm/swapfile: add percpu_ref support for swap

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@...nel.org> writes:
>>
>>> On Wed, Apr 14, 2021 at 01:44:58PM +0800, Huang, Ying wrote:
>>>> Dennis Zhou <dennis@...nel.org> writes:
>>>>
>>>>> On Wed, Apr 14, 2021 at 11:59:03AM +0800, Huang, Ying wrote:
>>>>>> Dennis Zhou <dennis@...nel.org> writes:
>>>>>>
>>>>>>> Hello,
>>>>>>>
>>>>>>> On Wed, Apr 14, 2021 at 10:06:48AM +0800, Huang, Ying wrote:
>>>>>>>> Miaohe Lin <linmiaohe@...wei.com> writes:
>>>>>>>>
>>>>>>>>> On 2021/4/14 9:17, Huang, Ying wrote:
>>>>>>>>>> Miaohe Lin <linmiaohe@...wei.com> writes:
>>>>>>>>>>
>>>>>>>>>>> On 2021/4/12 15:24, Huang, Ying wrote:
>>>>>>>>>>>> "Huang, Ying" <ying.huang@...el.com> writes:
>>>>>>>>>>>>
>>>>>>>>>>>>> Miaohe Lin <linmiaohe@...wei.com> 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@...wei.com>
>>>>>>>>>>>>>> ---
>>>>>>>>>>>>>>  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@linux.vnet.ibm.com/
>>>>>>>>>>>>
>>>>>>>>>>>> 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! :)

> 
>> 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
> 
> .
> 

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