Date:   Fri, 16 Apr 2021 08:54:53 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Dennis Zhou <dennis@...nel.org>
Cc:     Miaohe Lin <linmiaohe@...wei.com>, <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

Dennis Zhou <dennis@...nel.org> writes:

> 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?

Yes.  This looks good!  Thanks a lot!

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

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