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

["Huang, Ying" <ying.huang@intel.com>]

Miaohe Lin <linmiaohe@huawei.com> writes:

> 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?

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.

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);
>>>> .
>>>>
>> .
>>