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Message-ID: <ccf4107d-9dd5-4515-564a-8cbe43ca459f@huawei.com>
Date: Wed, 21 Apr 2021 09:59:58 +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>, <willy@...radead.org>, <minchan@...nel.org>,
<richard.weiyang@...il.com>, <shy828301@...il.com>,
<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH v3 1/4] mm/swapfile: use percpu_ref to serialize against
concurrent swapoff
On 2021/4/21 9:05, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@...wei.com> writes:
>
>> Using current get/put_swap_device() to guard against concurrent swapoff
>> for some swap ops, e.g. swap_readpage(), looks terrible because they
>> might take really long time. This patch adds the percpu_ref support to
>> serialize against concurrent swapoff. Also we 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 | 5 +--
>> mm/swapfile.c | 79 +++++++++++++++++++++++++++-----------------
>> 2 files changed, 52 insertions(+), 32 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 144727041e78..c9e7fea10b83 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 */
>> };
>> @@ -240,6 +239,7 @@ struct swap_cluster_list {
>> * The in-memory structure used to track swap areas.
>> */
>> struct swap_info_struct {
>> + struct percpu_ref users; /* indicate and keep swap device valid. */
>> 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 +260,7 @@ 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 */
>> @@ -511,7 +512,7 @@ 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 149e77454e3c..a640fc84be5b 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 +512,14 @@ 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);
>> +}
>> +
>> static void alloc_cluster(struct swap_info_struct *si, unsigned long idx)
>> {
>> struct swap_cluster_info *ci = si->cluster_info;
>> @@ -1270,18 +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()
>> @@ -1309,21 +1312,27 @@ 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 the si->users are checked before accessing other
>> + * fields of swap_info_struct.
>> + *
>> + * Paired with the spin_unlock() after setup_swap_info() in
>> + * enable_swap_info().
>> + */
>> + smp_rmb();
>> 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;
>> }
>>
>> @@ -2466,7 +2475,7 @@ 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;
>>
>> @@ -2497,10 +2506,9 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
>> spin_unlock(&p->lock);
>> spin_unlock(&swap_lock);
>> /*
>> - * Guarantee swap_map, cluster_info, etc. fields are valid
>> - * between get/put_swap_device() if SWP_VALID bit is set
>> + * Finished initialized swap device, now it's safe to reference it.
>
> s/initialized/initializing/
>
> Otherwise looks good to me! Thanks!
>
> Reviewed-by: "Huang, Ying" <ying.huang@...el.com>
>
Many thanks for your quick review and respond again. All these comments look good. Will fix it
in V4. Thank you very much !
>> */
>> - synchronize_rcu();
>> + percpu_ref_resurrect(&p->users);
>> spin_lock(&swap_lock);
>> spin_lock(&p->lock);
>> _enable_swap_info(p);
>> @@ -2616,16 +2624,16 @@ 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);
>> /*
>> - * wait for swap operations protected by get/put_swap_device()
>> - * to complete
>> + * Wait for swap operations protected by get/put_swap_device()
>> + * to complete.
>> + *
>> + * We need synchronize_rcu() here to protect the accessing to
>> + * the swap cache data structure.
>> */
>> + percpu_ref_kill(&p->users);
>> synchronize_rcu();
>> + wait_for_completion(&p->comp);
>>
>> flush_work(&p->discard_work);
>>
>> @@ -2857,6 +2865,12 @@ static struct swap_info_struct *alloc_swap_info(void)
>> if (!p)
>> return ERR_PTR(-ENOMEM);
>>
>> + if (percpu_ref_init(&p->users, swap_users_ref_free,
>> + PERCPU_REF_INIT_DEAD, GFP_KERNEL)) {
>> + kvfree(p);
>> + return ERR_PTR(-ENOMEM);
>> + }
>> +
>> spin_lock(&swap_lock);
>> for (type = 0; type < nr_swapfiles; type++) {
>> if (!(swap_info[type]->flags & SWP_USED))
>> @@ -2864,6 +2878,7 @@ static struct swap_info_struct *alloc_swap_info(void)
>> }
>> if (type >= MAX_SWAPFILES) {
>> spin_unlock(&swap_lock);
>> + percpu_ref_exit(&p->users);
>> kvfree(p);
>> return ERR_PTR(-EPERM);
>> }
>> @@ -2891,9 +2906,13 @@ static struct swap_info_struct *alloc_swap_info(void)
>> plist_node_init(&p->avail_lists[i], 0);
>> p->flags = SWP_USED;
>> spin_unlock(&swap_lock);
>> - kvfree(defer);
>> + if (defer) {
>> + percpu_ref_exit(&defer->users);
>> + kvfree(defer);
>> + }
>> spin_lock_init(&p->lock);
>> spin_lock_init(&p->cont_lock);
>> + init_completion(&p->comp);
>>
>> return p;
>> }
> .
>
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