Date:   Wed, 14 Apr 2021 09:17:47 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Miaohe Lin <linmiaohe@...wei.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

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.

>   ...
>
> 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?

Then we will depends on the implementation of percpu_ref.  If it changed
its implementation, it may take long to find out we need to change the
code here.  I guess in most cases, even adding a synchronize_rcu() here,
we still only need to wait for one grace period.  So the overhead to
call synchronize_rcu() is low here.  And the code is easier to be
maintained.

Best Regards,
Huang, Ying

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