Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
> We will use percpu-refcount to serialize against concurrent swapoff. This
> patch adds the percpu_ref support for later fixup.
>
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> ---
> include/linux/swap.h | 2 ++
> mm/swapfile.c | 25 ++++++++++++++++++++++---
> 2 files changed, 24 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 144727041e78..849ba5265c11 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -240,6 +240,7 @@ struct swap_cluster_list {
> * The in-memory structure used to track swap areas.
> */
> struct swap_info_struct {
> + struct percpu_ref users; /* serialization against concurrent swapoff */
> 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 +261,7 @@ 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 */
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 149e77454e3c..724173cd7d0c 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 +512,15 @@ 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);
> + percpu_ref_exit(&si->users);
Because percpu_ref_exit() is used, we cannot use percpu_ref_tryget() in
get_swap_device(), better to add comments there.
> +}
> +
> static void alloc_cluster(struct swap_info_struct *si, unsigned long idx)
> {
> struct swap_cluster_info *ci = si->cluster_info;
> @@ -2500,7 +2510,7 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
> * Guarantee swap_map, cluster_info, etc. fields are valid
> * between get/put_swap_device() if SWP_VALID bit is set
> */
> - synchronize_rcu();
> + percpu_ref_reinit(&p->users);
Although the effect is same, I think it's better to use
percpu_ref_resurrect() here to improve code readability.
> spin_lock(&swap_lock);
> spin_lock(&p->lock);
> _enable_swap_info(p);
> @@ -2621,11 +2631,13 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
> p->flags &= ~SWP_VALID; /* mark swap device as invalid */
> spin_unlock(&p->lock);
> spin_unlock(&swap_lock);
> +
> + percpu_ref_kill(&p->users);
> /*
> * wait for swap operations protected by get/put_swap_device()
> * to complete
> */
> - synchronize_rcu();
> + wait_for_completion(&p->comp);
Better to move percpu_ref_kill() after the comments. And maybe revise
the comments.
>
> flush_work(&p->discard_work);
>
> @@ -3132,7 +3144,7 @@ static bool swap_discardable(struct swap_info_struct *si)
> SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> {
> struct swap_info_struct *p;
> - struct filename *name;
> + struct filename *name = NULL;
> struct file *swap_file = NULL;
> struct address_space *mapping;
> int prio;
> @@ -3163,6 +3175,12 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>
> INIT_WORK(&p->discard_work, swap_discard_work);
>
> + init_completion(&p->comp);
> + error = percpu_ref_init(&p->users, swap_users_ref_free,
> + PERCPU_REF_INIT_DEAD, GFP_KERNEL);
> + if (unlikely(error))
> + goto bad_swap;
> +
> name = getname(specialfile);
> if (IS_ERR(name)) {
> error = PTR_ERR(name);
> @@ -3356,6 +3374,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> bad_swap_unlock_inode:
> inode_unlock(inode);
> bad_swap:
> + percpu_ref_exit(&p->users);
Usually the resource freeing order matches their allocating order
reversely. So, if there's no special reason, please follow that rule.
Best Regards,
Huang, Ying
> free_percpu(p->percpu_cluster);
> p->percpu_cluster = NULL;
> free_percpu(p->cluster_next_cpu);
