Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
> We will use percpu-refcount to serialize against concurrent swapoff. This
> patch adds the percpu_ref support for swap.
>
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> ---
> include/linux/swap.h | 3 +++
> mm/swapfile.c | 33 +++++++++++++++++++++++++++++----
> 2 files changed, 32 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 144727041e78..8be36eb58b7a 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 */
The comments aren't general enough. We use this to check whether the
swap device has been fully initialized, etc. May be something as below?
/* 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 +261,8 @@ 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 */
> + bool ref_initialized; /* 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..66515a3a2824 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;
> @@ -2500,7 +2509,7 @@ 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();
You cannot remove this without changing get/put_swap_device(). It's
better to squash at least PATCH 1-2.
> + percpu_ref_resurrect(&p->users);
> spin_lock(&swap_lock);
> spin_lock(&p->lock);
> _enable_swap_info(p);
> @@ -2621,11 +2630,18 @@ 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
> + * We need synchronize_rcu() here to protect the accessing
> + * to the swap cache data structure.
> */
> synchronize_rcu();
> + /*
> + * Wait for swap operations protected by get/put_swap_device()
> + * to complete.
> + */
I think the comments (after some revision) can be moved before
percpu_ref_kill(). The synchronize_rcu() comments can be merged.
> + wait_for_completion(&p->comp);
>
> flush_work(&p->discard_work);
>
> @@ -3132,7 +3148,7 @@ 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 +3179,15 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
>
> INIT_WORK(&p->discard_work, swap_discard_work);
>
> + if (!p->ref_initialized) {
I don't think it's necessary to add another flag p->ref_initialized. We
can distinguish newly allocated and reused swap_info_struct in alloc_swap_info().
Best Regards,
Huang, Ying
> + error = percpu_ref_init(&p->users, swap_users_ref_free,
> + PERCPU_REF_INIT_DEAD, GFP_KERNEL);
> + if (unlikely(error))
> + goto bad_swap;
> + init_completion(&p->comp);
> + p->ref_initialized = true;
> + }
> +
> name = getname(specialfile);
> if (IS_ERR(name)) {
> error = PTR_ERR(name);
