Miaohe Lin <linmiaohe@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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 */
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
