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

[Huang, Ying]

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().

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);
>> .
>>