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

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

On Thu, Feb 20, 2025 at 07:18:40PM +0000, Yosry Ahmed wrote:
> 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?

Never mind we keep the spinlock held on write locking.