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@xxxxxxxx>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
> ---
> 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())
