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Message-ID: <16a20b86-b690-9397-def1-1171828c245e@huawei.com>
Date: Mon, 19 Apr 2021 14:57:13 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: "Huang, Ying" <ying.huang@...el.com>
CC: <akpm@...ux-foundation.org>, <dennis@...nel.org>,
<tim.c.chen@...ux.intel.com>, <hughd@...gle.com>,
<hannes@...xchg.org>, <mhocko@...e.com>, <iamjoonsoo.kim@....com>,
<alexs@...nel.org>, <david@...hat.com>, <minchan@...nel.org>,
<richard.weiyang@...il.com>, <linux-kernel@...r.kernel.org>,
<linux-mm@...ck.org>
Subject: Re: [PATCH v2 2/5] mm/swapfile: use percpu_ref to serialize against
concurrent swapoff
On 2021/4/19 10:54, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@...wei.com> writes:
>
>> Use percpu_ref to serialize against concurrent swapoff. Also remove the
>> SWP_VALID flag because it's used together with RCU solution.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
>> include/linux/swap.h | 3 +--
>> mm/swapfile.c | 43 +++++++++++++++++--------------------------
>> 2 files changed, 18 insertions(+), 28 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 8be36eb58b7a..993693b38109 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -177,7 +177,6 @@ enum {
>> SWP_PAGE_DISCARD = (1 << 10), /* freed swap page-cluster discards */
>> SWP_STABLE_WRITES = (1 << 11), /* no overwrite PG_writeback pages */
>> SWP_SYNCHRONOUS_IO = (1 << 12), /* synchronous IO is efficient */
>> - SWP_VALID = (1 << 13), /* swap is valid to be operated on? */
>> /* add others here before... */
>> SWP_SCANNING = (1 << 14), /* refcount in scan_swap_map */
>> };
>> @@ -514,7 +513,7 @@ sector_t swap_page_sector(struct page *page);
>>
>> static inline void put_swap_device(struct swap_info_struct *si)
>> {
>> - rcu_read_unlock();
>> + percpu_ref_put(&si->users);
>> }
>>
>> #else /* CONFIG_SWAP */
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 66515a3a2824..90e197bc2eeb 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -1279,18 +1279,12 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
>> * via preventing the swap device from being swapoff, until
>> * put_swap_device() is called. Otherwise return NULL.
>> *
>> - * The entirety of the RCU read critical section must come before the
>> - * return from or after the call to synchronize_rcu() in
>> - * enable_swap_info() or swapoff(). So if "si->flags & SWP_VALID" is
>> - * true, the si->map, si->cluster_info, etc. must be valid in the
>> - * critical section.
>> - *
>> * Notice that swapoff or swapoff+swapon can still happen before the
>> - * rcu_read_lock() in get_swap_device() or after the rcu_read_unlock()
>> - * in put_swap_device() if there isn't any other way to prevent
>> - * swapoff, such as page lock, page table lock, etc. The caller must
>> - * be prepared for that. For example, the following situation is
>> - * possible.
>> + * percpu_ref_tryget_live() in get_swap_device() or after the
>> + * percpu_ref_put() in put_swap_device() if there isn't any other way
>> + * to prevent swapoff, such as page lock, page table lock, etc. The
>> + * caller must be prepared for that. For example, the following
>> + * situation is possible.
>> *
>> * CPU1 CPU2
>> * do_swap_page()
>> @@ -1318,21 +1312,24 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
>> si = swp_swap_info(entry);
>> if (!si)
>> goto bad_nofile;
>> -
>> - rcu_read_lock();
>> - if (data_race(!(si->flags & SWP_VALID)))
>> - goto unlock_out;
>> + if (!percpu_ref_tryget_live(&si->users))
>> + goto out;
>> + /*
>> + * Guarantee we will not reference uninitialized fields
>> + * of swap_info_struct.
>> + */
>
> /*
> * Guarantee the si->users are checked before accessing other fields of
> * swap_info_struct.
> */
>
>> + smp_rmb();
>
> Usually, smp_rmb() need to be paired with smp_wmb(). Some comments are
> needed for that. Here smb_rmb() is paired with the spin_unlock() after
> setup_swap_info() in enable_swap_info().
>
>> offset = swp_offset(entry);
>> if (offset >= si->max)
>> - goto unlock_out;
>> + goto put_out;
>>
>> return si;
>> bad_nofile:
>> pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
>> out:
>> return NULL;
>> -unlock_out:
>> - rcu_read_unlock();
>> +put_out:
>> + percpu_ref_put(&si->users);
>> return NULL;
>> }
>>
>> @@ -2475,7 +2472,7 @@ static void setup_swap_info(struct swap_info_struct *p, int prio,
>>
>> static void _enable_swap_info(struct swap_info_struct *p)
>> {
>> - p->flags |= SWP_WRITEOK | SWP_VALID;
>> + p->flags |= SWP_WRITEOK;
>> atomic_long_add(p->pages, &nr_swap_pages);
>> total_swap_pages += p->pages;
>>
>> @@ -2507,7 +2504,7 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
>> spin_unlock(&swap_lock);
>> /*
>> * Guarantee swap_map, cluster_info, etc. fields are valid
>> - * between get/put_swap_device() if SWP_VALID bit is set
>> + * between get/put_swap_device().
>> */
>
> The comments need to be revised. Something likes below?
>
> /* Finished initialized swap device, now it's safe to reference it */
>
All look good for me. Will do. Many thanks!
> Best Regards,
> Huang, Ying
>
>> percpu_ref_resurrect(&p->users);
>> spin_lock(&swap_lock);
>> @@ -2625,12 +2622,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>>
>> reenable_swap_slots_cache_unlock();
>>
>> - spin_lock(&swap_lock);
>> - spin_lock(&p->lock);
>> - p->flags &= ~SWP_VALID; /* mark swap device as invalid */
>> - spin_unlock(&p->lock);
>> - spin_unlock(&swap_lock);
>> -
>> percpu_ref_kill(&p->users);
>> /*
>> * We need synchronize_rcu() here to protect the accessing
>
> .
>
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