Re: [PATCH v7 11/17] zsmalloc: make zspage lock preemptible

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

On Fri, Feb 21, 2025 at 06:38:04PM +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 | 189 +++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 135 insertions(+), 54 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 1424ee73cbb5..250f1fddaf34 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 +294,91 @@ struct mapping_area {
>  	enum zs_mapmode vm_mm; /* mapping mode */
>  };
>  
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> +#define zsl_dep_map(zsl) (&(zsl)->dep_map)
> +#else
> +#define zsl_dep_map(zsl) NULL
> +#endif
> +
> +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);

Can't we remove this #ifdef as well by adding a similar macro? (e.g.
zsl_lock_class())