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

[Dennis Zhou]

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?

> >
> > Many thanks.
> > But We may still need to add a smp_rmb() in get_swap_device() in case
> > we can't add ACQUIRE for refcount.
> 
> Yes.
> 
> Best Regards,
> Huang, Ying
> 

Thanks,
Dennis