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@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?
Never mind we keep the spinlock held on write locking.
