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Subject: Re: [PATCH 1/5] mm/swapfile: add percpu_ref support for swap
To:     Dennis Zhou <dennis@kernel.org>,
        "Huang, Ying" <ying.huang@intel.com>
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From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <450d2a80-f5e0-2eb2-2dad-ddc56de49560@huawei.com>
Date:   Thu, 15 Apr 2021 11:16:42 +0800
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On 2021/4/14 22:53, Dennis Zhou wrote:
> On Wed, Apr 14, 2021 at 01:44:58PM +0800, Huang, Ying wrote:
>> Dennis Zhou <dennis@kernel.org> writes:
>>
>>> On Wed, Apr 14, 2021 at 11:59:03AM +0800, Huang, Ying wrote:
>>>> Dennis Zhou <dennis@kernel.org> writes:
>>>>
>>>>> Hello,
>>>>>
>>>>> On Wed, Apr 14, 2021 at 10:06:48AM +0800, Huang, Ying wrote:
>>>>>> Miaohe Lin <linmiaohe@huawei.com> writes:
>>>>>>
>>>>>>> On 2021/4/14 9:17, Huang, Ying wrote:
>>>>>>>> Miaohe Lin <linmiaohe@huawei.com> writes:
>>>>>>>>
>>>>>>>>> On 2021/4/12 15:24, Huang, Ying wrote:
>>>>>>>>>> "Huang, Ying" <ying.huang@intel.com> writes:
>>>>>>>>>>
>>>>>>>>>>> Miaohe Lin <linmiaohe@huawei.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@huawei.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

This assumption might not be held for swap. If we can get a ref, that means
the object hasn't been destroyed or the object has been destroyed and created
again. It's because swp_entry can hold a really long time while swapoff+swapon
happened. So we may get a ref to a newly swapon-ed swap device using old swap_entry.
So we must guarantee that we will not reference uninitialized fields of newly
swapon-ed swap device.

Does this make sense for you? Thanks.

> 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.
> 
> Thanks,
> Dennis
> 
> .
>