On Fri, Feb 14, 2025 at 01:50:23PM +0900, Sergey Senozhatsky wrote:
> In order to implement preemptible object mapping we need a zspage lock
> that satisfies several preconditions:
> - it should be reader-write type of a lock
> - it should be possible to hold it from any context, but also being
> preemptible if the context allows it
> - we never sleep while acquiring but can sleep while holding in read
> mode
>
> An rwsemaphore doesn't suffice, due to atomicity requirements, rwlock
> doesn't satisfy due to reader-preemptability requirement. It's also
> worth to mention, that per-zspage rwsem is a little too memory heavy
> (we can easily have double digits megabytes used only on rwsemaphores).
>
> Switch over from rwlock_t to a atomic_t-based implementation of a
> reader-writer semaphore that satisfies all of the preconditions.
>
> The spin-lock based zspage_lock is suggested by Hillf Danton.
>
> Suggested-by: Hillf Danton <hdanton@xxxxxxxx>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
> ---
> mm/zsmalloc.c | 246 +++++++++++++++++++++++++++++++++++++++-----------
> 1 file changed, 192 insertions(+), 54 deletions(-)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 2e338cde0d21..bc679a3e1718 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -226,6 +226,9 @@ struct zs_pool {
> /* protect zspage migration/compaction */
> rwlock_t lock;
> atomic_t compaction_in_progress;
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> + struct lock_class_key lock_class;
> +#endif
> };
>
> static inline void zpdesc_set_first(struct zpdesc *zpdesc)
> @@ -257,6 +260,18 @@ static inline void free_zpdesc(struct zpdesc *zpdesc)
> __free_page(page);
> }
>
> +#define ZS_PAGE_UNLOCKED 0
> +#define ZS_PAGE_WRLOCKED -1
> +
> +struct zspage_lock {
> + spinlock_t lock;
> + int cnt;
> +
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> + struct lockdep_map dep_map;
> +#endif
> +};
> +
> struct zspage {
> struct {
> unsigned int huge:HUGE_BITS;
> @@ -269,7 +284,7 @@ struct zspage {
> struct zpdesc *first_zpdesc;
> struct list_head list; /* fullness list */
> struct zs_pool *pool;
> - rwlock_t lock;
> + struct zspage_lock zsl;
> };
>
> struct mapping_area {
> @@ -279,6 +294,148 @@ struct mapping_area {
> enum zs_mapmode vm_mm; /* mapping mode */
> };
>
> +static void zspage_lock_init(struct zspage *zspage)
> +{
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> + lockdep_init_map(&zspage->zsl.dep_map, "zspage->lock",
> + &zspage->pool->lock_class, 0);
> +#endif
> +
> + spin_lock_init(&zspage->zsl.lock);
> + zspage->zsl.cnt = ZS_PAGE_UNLOCKED;
> +}
> +
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
Instead of the #ifdef and repeating all the functions, can't we do
something like:
#ifdef CONFIG_DEBUG_LOCK_ALLOC
#define zspage_lock_map(zsl) (&zsl->dep_map)
#else
#define zspage_lock_map(zsl) /* empty or NULL */
#endif
Then we can just have one version of the functions and use
zspage_lock_map() instead of zsl->dep_map, right?
> +static inline void __read_lock(struct zspage *zspage)
> +{
> + struct zspage_lock *zsl = &zspage->zsl;
> +
> + rwsem_acquire_read(&zsl->dep_map, 0, 0, _RET_IP_);
> +
> + spin_lock(&zsl->lock);
> + zsl->cnt++;
Shouldn't we check if the lock is write locked?
> + spin_unlock(&zsl->lock);
> +
> + lock_acquired(&zsl->dep_map, _RET_IP_);
> +}
> +
> +static inline void __read_unlock(struct zspage *zspage)
> +{
> + struct zspage_lock *zsl = &zspage->zsl;
> +
> + rwsem_release(&zsl->dep_map, _RET_IP_);
> +
> + spin_lock(&zsl->lock);
> + zsl->cnt--;
> + spin_unlock(&zsl->lock);
> +}
