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

[Huang, Ying]

Dennis Zhou <dennis@xxxxxxxxxx> writes:

> On Wed, Apr 14, 2021 at 11:59:03AM +0800, Huang, Ying wrote:
>> Dennis Zhou <dennis@xxxxxxxxxx> writes:
>> 
>> > Hello,
>> >
>> > On Wed, Apr 14, 2021 at 10:06:48AM +0800, Huang, Ying wrote:
>> >> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>> >> 
>> >> > On 2021/4/14 9:17, Huang, Ying wrote:
>> >> >> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>> >> >> 
>> >> >>> On 2021/4/12 15:24, Huang, Ying wrote:
>> >> >>>> "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
>> >> >>>>
>> >> >>>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> 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@xxxxxxxxxx>
>> >> >>>>>> ---
>> >> >>>>>>  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@xxxxxxxxxxxxxxxxxx/
>> >> >>>>
>> >> >>>> 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.
>> >
>> > I'm just catching up and following along a little bit. I apologize I
>> > haven't read the swap code, but my understanding is you are trying to
>> > narrow a race condition with swapoff. That makes sense to me. I'm not
>> > sure I follow the need to race with reinitializing the ref though? Is it
>> > not possible to wait out the dying swap info and then create a new one
>> > rather than push acquire semantics?
>> 
>> We want to check whether the swap entry is valid (that is, the swap
>> device isn't swapped off now), prevent it from swapping off, then access
>> the swap_info_struct data structure.  When accessing swap_info_struct,
>> we want to guarantee the ordering, so that we will not reference
>> uninitialized fields of swap_info_struct.
>> 
>
> So in the normal context of percpu_ref, once someone can access it, the
> elements that it is protecting are expected to be initialized.

If we can make sure that all elements being initialized fully, why not
just use percpu_ref_get() instead of percpu_ref_tryget*()?

> In the basic case for swap off, I'm seeing the goal as to prevent
> destruction until anyone currently accessing swap is done. In this
> case wouldn't we always be protecting a live struct?
>
> I'm maybe not understanding what conditions you're trying to revive the
> percpu_ref?

A swap entry likes an indirect pointer to a swap device.  We may hold a
swap entry for long time, so that the swap device is swapoff/swapon.
Then we need to make sure the swap device are fully initialized before
accessing the swap device via the swap entry.

Best Regards,
Huang, Ying