From: Dennis Zhou <dennis@kernel.org>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>,
akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@suse.com,
iamjoonsoo.kim@lge.com, vbabka@suse.cz,
alex.shi@linux.alibaba.com, willy@infradead.org,
minchan@kernel.org, richard.weiyang@gmail.com, hughd@google.com,
tim.c.chen@linux.intel.com, linux-kernel@vger.kernel.org,
linux-mm@kvack.org
Subject: Re: [PATCH 1/5] mm/swapfile: add percpu_ref support for swap
Date: Wed, 14 Apr 2021 03:44:37 +0000 [thread overview]
Message-ID: <YHZlJZh8T4ZhLhp6@google.com> (raw)
In-Reply-To: <87tuo9sjpj.fsf@yhuang6-desk1.ccr.corp.intel.com>
Hello,
On Wed, Apr 14, 2021 at 10:06:48AM +0800, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@huawei.com> writes:
>
> > On 2021/4/14 9:17, Huang, Ying wrote:
> >> Miaohe Lin <linmiaohe@huawei.com> writes:
> >>
> >>> On 2021/4/12 15:24, Huang, Ying wrote:
> >>>> "Huang, Ying" <ying.huang@intel.com> writes:
> >>>>
> >>>>> Miaohe Lin <linmiaohe@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 <linmiaohe@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.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?
> >
> > 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
next prev parent reply other threads:[~2021-04-14 3:44 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-08 13:08 [PATCH 0/5] close various race windows for swap Miaohe Lin
2021-04-08 13:08 ` [PATCH 1/5] mm/swapfile: add percpu_ref support " Miaohe Lin
2021-04-12 3:30 ` Huang, Ying
2021-04-12 6:59 ` Miaohe Lin
2021-04-12 7:24 ` Huang, Ying
2021-04-13 12:39 ` Miaohe Lin
2021-04-14 1:17 ` Huang, Ying
2021-04-14 1:58 ` Miaohe Lin
2021-04-14 2:06 ` Huang, Ying
2021-04-14 3:44 ` Dennis Zhou [this message]
2021-04-14 3:59 ` Huang, Ying
2021-04-14 4:05 ` Dennis Zhou
2021-04-14 5:44 ` Huang, Ying
2021-04-14 14:53 ` Dennis Zhou
2021-04-15 3:16 ` Miaohe Lin
2021-04-15 4:20 ` Dennis Zhou
2021-04-15 9:17 ` Miaohe Lin
2021-04-15 5:24 ` Huang, Ying
2021-04-15 14:31 ` Dennis Zhou
2021-04-16 0:54 ` Huang, Ying
2021-04-16 2:27 ` Miaohe Lin
2021-04-16 6:25 ` Huang, Ying
2021-04-16 8:30 ` Miaohe Lin
2021-04-08 13:08 ` [PATCH 2/5] swap: fix do_swap_page() race with swapoff Miaohe Lin
2021-04-08 21:34 ` Tim Chen
2021-04-09 8:42 ` Miaohe Lin
2021-04-09 17:17 ` Tim Chen
2021-04-10 3:17 ` Miaohe Lin
2021-04-12 1:44 ` Huang, Ying
2021-04-12 3:24 ` Miaohe Lin
2021-04-13 1:27 ` Huang, Ying
2021-04-13 19:24 ` Tim Chen
2021-04-14 1:04 ` Huang, Ying
2021-04-14 2:20 ` Miaohe Lin
2021-04-14 16:13 ` Tim Chen
2021-04-15 3:19 ` Miaohe Lin
2021-04-14 2:55 ` Miaohe Lin
2021-04-14 3:07 ` Huang, Ying
2021-04-14 3:27 ` Miaohe Lin
2021-04-08 13:08 ` [PATCH 3/5] mm/swap_state: fix get_shadow_from_swap_cache() " Miaohe Lin
2021-04-13 1:33 ` Huang, Ying
2021-04-14 2:42 ` Miaohe Lin
2021-04-08 13:08 ` [PATCH 4/5] mm/swap_state: fix potential faulted in race in swap_ra_info() Miaohe Lin
2021-04-09 8:50 ` Huang, Ying
2021-04-09 9:00 ` Miaohe Lin
2021-04-12 0:55 ` Huang, Ying
2021-04-12 3:17 ` Miaohe Lin
2021-04-08 13:08 ` [PATCH 5/5] mm/swap_state: fix swap_cluster_readahead() race with swapoff Miaohe Lin
2021-04-13 1:36 ` Huang, Ying
2021-04-14 2:43 ` Miaohe Lin
2021-04-08 14:55 ` [PATCH 0/5] close various race windows for swap riteshh
2021-04-09 8:01 ` Miaohe Lin
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