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Subject: Re: [PATCH v2 1/5] mm/swapfile: add percpu_ref support for swap
To:     "Huang, Ying" <ying.huang@intel.com>
CC:     <akpm@linux-foundation.org>, <dennis@kernel.org>,
        <tim.c.chen@linux.intel.com>, <hughd@google.com>,
        <hannes@cmpxchg.org>, <mhocko@suse.com>, <iamjoonsoo.kim@lge.com>,
        <alexs@kernel.org>, <david@redhat.com>, <minchan@kernel.org>,
        <richard.weiyang@gmail.com>, <linux-kernel@vger.kernel.org>,
        <linux-mm@kvack.org>
References: <20210417094039.51711-1-linmiaohe@huawei.com>
 <20210417094039.51711-2-linmiaohe@huawei.com>
 <87eef7kmzw.fsf@yhuang6-desk1.ccr.corp.intel.com>
 <753f414f-34a1-b16a-f826-7deb2dcd4af6@huawei.com>
 <87czuq4uo2.fsf@yhuang6-desk1.ccr.corp.intel.com>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <dfbb8f36-e172-b682-fbfa-168a1ac2d456@huawei.com>
Date:   Mon, 19 Apr 2021 15:35:09 +0800
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On 2021/4/19 15:09, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@huawei.com> writes:
> 
>> On 2021/4/19 10:48, Huang, Ying wrote:
>>> Miaohe Lin <linmiaohe@huawei.com> writes:
>>>
>>>> We will use percpu-refcount to serialize against concurrent swapoff. This
>>>> patch adds the percpu_ref support for swap.
>>>>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>> ---
>>>>  include/linux/swap.h |  3 +++
>>>>  mm/swapfile.c        | 33 +++++++++++++++++++++++++++++----
>>>>  2 files changed, 32 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>>>> index 144727041e78..8be36eb58b7a 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 */
>>>
>>> The comments aren't general enough.  We use this to check whether the
>>> swap device has been fully initialized, etc. May be something as below?
>>>
>>> /* indicate and keep swap device valid */
>>
>> Looks good.
>>
>>>
>>>>  	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 +261,8 @@ 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 */
>>>> +	bool ref_initialized;		/* 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..66515a3a2824 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 +512,14 @@ 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);
>>>> +}
>>>> +
>>>>  static void alloc_cluster(struct swap_info_struct *si, unsigned long idx)
>>>>  {
>>>>  	struct swap_cluster_info *ci = si->cluster_info;
>>>> @@ -2500,7 +2509,7 @@ 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();
>>>
>>> You cannot remove this without changing get/put_swap_device().  It's
>>> better to squash at least PATCH 1-2.
>>
>> Will squash PATCH 1-2. Thanks.
>>
>>>
>>>> +	percpu_ref_resurrect(&p->users);
>>>>  	spin_lock(&swap_lock);
>>>>  	spin_lock(&p->lock);
>>>>  	_enable_swap_info(p);
>>>> @@ -2621,11 +2630,18 @@ 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
>>>> +	 * We need synchronize_rcu() here to protect the accessing
>>>> +	 * to the swap cache data structure.
>>>>  	 */
>>>>  	synchronize_rcu();
>>>> +	/*
>>>> +	 * Wait for swap operations protected by get/put_swap_device()
>>>> +	 * to complete.
>>>> +	 */
>>>
>>> I think the comments (after some revision) can be moved before
>>> percpu_ref_kill().  The synchronize_rcu() comments can be merged.
>>>
>>
>> Ok.
>>
>>>> +	wait_for_completion(&p->comp);
>>>>  
>>>>  	flush_work(&p->discard_work);
>>>>  
>>>> @@ -3132,7 +3148,7 @@ static bool swap_discardable(struct swap_info_struct *si)
>>>>  SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>>>>  {
>>>>  	struct swap_info_struct *p;
>>>> -	struct filename *name;
>>>> +	struct filename *name = NULL;
>>>>  	struct file *swap_file = NULL;
>>>>  	struct address_space *mapping;
>>>>  	int prio;
>>>> @@ -3163,6 +3179,15 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>>>>  
>>>>  	INIT_WORK(&p->discard_work, swap_discard_work);
>>>>  
>>>> +	if (!p->ref_initialized) {
>>>
>>> I don't think it's necessary to add another flag p->ref_initialized.  We
>>> can distinguish newly allocated and reused swap_info_struct in alloc_swap_info().
>>>
>>
>> If newly allocated swap_info_struct failed to init percpu_ref, it will be considered as
>> a reused one in alloc_swap_info() _but_ the field users of swap_info_struct is actually
>> uninitialized. Does this make sense for you?
> 
> We can call percpu_ref_init() just after kvzalloc() in alloc_swap_info().
> 

Yes, we can do it this way. But using ref_initialized might make the code more straightforward
and simple?

> Best Regards,
> Huang, Ying
> 
>> Many Thanks for quick review.
>>
>>> Best Regards,
>>> Huang, Ying
>>>
>>>> +		error = percpu_ref_init(&p->users, swap_users_ref_free,
>>>> +					PERCPU_REF_INIT_DEAD, GFP_KERNEL);
>>>> +		if (unlikely(error))
>>>> +			goto bad_swap;
>>>> +		init_completion(&p->comp);
>>>> +		p->ref_initialized = true;
>>>> +	}
>>>> +
>>>>  	name = getname(specialfile);
>>>>  	if (IS_ERR(name)) {
>>>>  		error = PTR_ERR(name);
>>> .
>>>
> .
>