Miaohe Lin <linmiaohe@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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@xxxxxxxxx>
> */
> - 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;
> }
