Re: [PATCH v2 1/5] mm/swapfile: add percpu_ref support for swap

[Miaohe Lin]

On 2021/4/19 15:52, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
> 
>> On 2021/4/19 15:09, Huang, Ying wrote:
>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>>>
>>>> On 2021/4/19 10:48, Huang, Ying wrote:
>>>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> 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@xxxxxxxxxx>
>>>>>> ---
>>>>>>  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?
> 
> I think that it's simpler to call percpu_ref_init() in
> alloc_swap_info().  We can just call percpu_ref_init() for allocated
> swap_info_struct blindly, and call percpu_ref_exit() if we reuse.
> 

Looks good. Will do. Many thanks again.

> Best Regards,
> Huang, Ying
> 
>>> 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);
>>>>> .
>>>>>
>>> .
>>>
> .
>