Message-ID: <YHcB36r5gOyXnozT@google.com>
Date:   Wed, 14 Apr 2021 14:53:19 +0000
From:   Dennis Zhou <dennis@...nel.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org,
        hannes@...xchg.org, mhocko@...e.com, iamjoonsoo.kim@....com,
        vbabka@...e.cz, alex.shi@...ux.alibaba.com, willy@...radead.org,
        minchan@...nel.org, richard.weiyang@...il.com, hughd@...gle.com,
        tim.c.chen@...ux.intel.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 1/5] mm/swapfile: add percpu_ref support for swap

On Wed, Apr 14, 2021 at 01:44:58PM +0800, Huang, Ying wrote:
> Dennis Zhou <dennis@...nel.org> writes:
> 
> > On Wed, Apr 14, 2021 at 11:59:03AM +0800, Huang, Ying wrote:
> >> Dennis Zhou <dennis@...nel.org> writes:
> >> 
> >> > Hello,
> >> >
> >> > On Wed, Apr 14, 2021 at 10:06:48AM +0800, Huang, Ying wrote:
> >> >> Miaohe Lin <linmiaohe@...wei.com> writes:
> >> >> 
> >> >> > On 2021/4/14 9:17, Huang, Ying wrote:
> >> >> >> Miaohe Lin <linmiaohe@...wei.com> writes:
> >> >> >> 
> >> >> >>> On 2021/4/12 15:24, Huang, Ying wrote:
> >> >> >>>> "Huang, Ying" <ying.huang@...el.com> writes:
> >> >> >>>>
> >> >> >>>>> Miaohe Lin <linmiaohe@...wei.com> 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@...wei.com>
> >> >> >>>>>> ---
> >> >> >>>>>>  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@linux.vnet.ibm.com/
> >> >> >>>>
> >> >> >>>> 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*()?
> 

Generally, the lookup is protected with rcu and then
percpu_ref_tryget*() is used to obtain a reference. percpu_ref_get() is
only good if you already have a ref as it increments regardless of being
0.

What I mean is if you can get a ref, that means the object hasn't been
destroyed. This differs from the semantics you are looking for which I
understand to be: I have long lived pointers to objects. The object may
die, but I may resurrect it and I want the old pointers to still be
valid.

When is it possible for someone to have a pointer to the swap device and
the refcount goes to 0? It might be better to avoid this situation than
add acquire semantics.

> > 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.
> 

So if I have some number of outstanding references, and then
percpu_ref_kill() is called, then only those that have the pointer will
be able to use the swap device as those references are still good. Prior
to calling percpu_ref_kill(), call_rcu() needs to be called on lookup
data structure.

My personal understanding of tryget() vs tryget_live() is that it
provides a 2 phase clean up and bounds the ability for new users to come
in (cgroup destruction is a primary user). As tryget() might inevitably
let a cgroup live long past its removal, tryget_live() will say oh
you're in the process of dying do something else.

Thanks,
Dennis

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