Re: [PATCH v6 11/17] zsmalloc: make zspage lock preemptible

[Yosry Ahmed <yosry.ahmed@linux.dev>]

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@sina.com>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> ---
>  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);
> +}