Hi:
On 2021/4/5 18:20, Miaohe Lin wrote:
> frontswap_register_ops can race with swapon. Consider the following scene:
Any comment or suggestion? Or is this race window too theoretical to fix?
Thanks.
>
> CPU1 CPU2
> ---- ----
> frontswap_register_ops
> fill bitmap a
> ops->init
> sys_swapon
> enable_swap_info
> frontswap_init without new ops
> add ops to frontswap_ops list
> check if swap_active_head changed
> add to swap_active_head
>
> So the frontswap_ops init is missed on the new swap device. Consider the
> another scene:
> CPU1 CPU2
> ---- ----
> frontswap_register_ops
> fill bitmap a
> ops->init
> add ops to frontswap_ops list
> sys_swapon
> enable_swap_info
> frontswap_init with new ops
> add to swap_active_head
> check if swap_active_head changed
> ops->init for new swap device [twice!]
>
> The frontswap_ops init will be called two times on the new swap device this
> time. frontswap_register_ops can also race with swapoff. Consider the
> following scene:
>
> CPU1 CPU2
> ---- ----
> sys_swapoff
> removed from swap_active_head
> frontswap_register_ops
> fill bitmap a
> ops->init without swap device
> add ops to frontswap_ops list
> invalidate_area with new ops
> check if swap_active_head changed
>
> We could call invalidate_area on a swap device under swapoff with frontswap
> is uninitialized yet. Fix all these by using swapon_mutex to guard against
> race with swapon and add swap_info_get_if_under_swapoff() to collect swap
> devices under swapoff.
>
> Fixes: d1dc6f1bcf1e ("frontswap: allow multiple backends")
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> ---
> include/linux/swapfile.h | 2 ++
> mm/frontswap.c | 40 +++++++++++++++++-----------------------
> mm/swapfile.c | 13 ++++++++++++-
> 3 files changed, 31 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/swapfile.h b/include/linux/swapfile.h
> index e06febf62978..7ae15d917828 100644
> --- a/include/linux/swapfile.h
> +++ b/include/linux/swapfile.h
> @@ -9,8 +9,10 @@
> extern spinlock_t swap_lock;
> extern struct plist_head swap_active_head;
> extern struct swap_info_struct *swap_info[];
> +extern struct mutex swapon_mutex;
> extern int try_to_unuse(unsigned int, bool, unsigned long);
> extern unsigned long generic_max_swapfile_size(void);
> extern unsigned long max_swapfile_size(void);
> +extern struct swap_info_struct *swap_info_get_if_under_swapoff(int type);
>
> #endif /* _LINUX_SWAPFILE_H */
> diff --git a/mm/frontswap.c b/mm/frontswap.c
> index 130e301c5ac0..c16bfc7550b5 100644
> --- a/mm/frontswap.c
> +++ b/mm/frontswap.c
> @@ -123,12 +123,26 @@ void frontswap_register_ops(struct frontswap_ops *ops)
>
> bitmap_zero(a, MAX_SWAPFILES);
> bitmap_zero(b, MAX_SWAPFILES);
> -
> + mutex_lock(&swapon_mutex);
> spin_lock(&swap_lock);
> plist_for_each_entry(si, &swap_active_head, list) {
> if (!WARN_ON(!si->frontswap_map))
> set_bit(si->type, a);
> }
> + /*
> + * There might be some swap devices under swapoff, i.e. they are
> + * removed from swap_active_head but frontswap_invalidate_area()
> + * is not called yet due to swapon_mutex is held here. We must
> + * collect these swap devices and call ops->init on them or they
> + * might invalidate frontswap area while frontswap is uninitialized.
> + */
> + for_each_clear_bit(i, a, MAX_SWAPFILES) {
> + si = swap_info_get_if_under_swapoff(i);
> + if (!si || !si->frontswap_map)
> + continue;
> + set_bit(si->type, b);
> + }
> + bitmap_or(a, a, b, MAX_SWAPFILES);
> spin_unlock(&swap_lock);
>
> /* the new ops needs to know the currently active swap devices */
> @@ -144,29 +158,9 @@ void frontswap_register_ops(struct frontswap_ops *ops)
> ops->next = frontswap_ops;
> } while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);
>
> - static_branch_inc(&frontswap_enabled_key);
> -
> - spin_lock(&swap_lock);
> - plist_for_each_entry(si, &swap_active_head, list) {
> - if (si->frontswap_map)
> - set_bit(si->type, b);
> - }
> - spin_unlock(&swap_lock);
> + mutex_unlock(&swapon_mutex);
>
> - /*
> - * On the very unlikely chance that a swap device was added or
> - * removed between setting the "a" list bits and the ops init
> - * calls, we re-check and do init or invalidate for any changed
> - * bits.
> - */
> - if (unlikely(!bitmap_equal(a, b, MAX_SWAPFILES))) {
> - for (i = 0; i < MAX_SWAPFILES; i++) {
> - if (!test_bit(i, a) && test_bit(i, b))
> - ops->init(i);
> - else if (test_bit(i, a) && !test_bit(i, b))
> - ops->invalidate_area(i);
> - }
> - }
> + static_branch_inc(&frontswap_enabled_key);
> }
> EXPORT_SYMBOL(frontswap_register_ops);
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 149e77454e3c..ee736533717f 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -89,7 +89,7 @@ static DEFINE_SPINLOCK(swap_avail_lock);
>
> struct swap_info_struct *swap_info[MAX_SWAPFILES];
>
> -static DEFINE_MUTEX(swapon_mutex);
> +DEFINE_MUTEX(swapon_mutex);
>
> static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
> /* Activity counter to indicate that a swapon or swapoff has occurred */
> @@ -2958,6 +2958,17 @@ __weak unsigned long max_swapfile_size(void)
> return generic_max_swapfile_size();
> }
>
> +struct swap_info_struct *swap_info_get_if_under_swapoff(int type)
> +{
> + struct swap_info_struct *si = swap_type_to_swap_info(type);
> +
> + if (!si || !si->swap_map)
> + return NULL;
> + if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK))
> + return si;
> + return NULL;
> +}
> +
> static unsigned long read_swap_header(struct swap_info_struct *p,
> union swap_header *swap_header,
> struct inode *inode)
>
