Miaohe Lin <linmia...@huawei.com> writes:
> On 2021/4/12 15:24, Huang, Ying wrote:
>> "Huang, Ying" <ying.hu...@intel.com> writes:
>>
>>> Miaohe Lin <linmia...@huawei.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 <linmia...@huawei.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.gb7...@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.
> ...
>
> There is indeed missing smp_rmb() in percpu_ref_tryget_live. So I think the
> above
> scene possible and we should fix this.
>
>>>> spin_lock(&swap_lock);
>>>> spin_lock(&p->lock);
>>>> _enable_swap_info(p);
>>>> @@ -2621,11 +2631,13 @@ 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
>>>> */
>>>> - synchronize_rcu();
>>>> + wait_for_completion(&p->comp);
>>>
>>> Better to move percpu_ref_kill() after the comments. And maybe revise
>>> the comments.
>>
>> After reading the original commit description as above, I found that we
>> need synchronize_rcu() here to protect the accessing to the swap cache
>> data structure. Because there's call_rcu() during percpu_ref_kill(), it
>> appears OK to keep the synchronize_rcu() here. And we need to revise
>> the comments to make it clear what is protected by which operation.
>>
>
> Per my understanding, percpu_ref->data->release is called only after the
> refcnt
> reaches 0, including a full grace period has elapsed or refcnt won't be 0.
> wait_for_completion() is used for waiting the last refcnt being released. So
> synchronize_rcu() is not necessary here?
Then we will depends on the implementation of percpu_ref. If it changed
its implementation, it may take long to find out we need to change the
code here. I guess in most cases, even adding a synchronize_rcu() here,
we still only need to wait for one grace period. So the overhead to
call synchronize_rcu() is low here. And the code is easier to be
maintained.
Best Regards,
Huang, Ying
