[RFC,4/7] mm: zswap: remove page reclaim logic from zsmalloc

  With the recent enhancement to zswap enabling direct page writeback, the
need for the shrink code in zsmalloc has become obsolete. As a result,
this commit removes the page reclaim logic from zsmalloc entirely.

Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
---
 mm/zsmalloc.c | 291 +-------------------------------------------------
 1 file changed, 2 insertions(+), 289 deletions(-)
  

On Mon, Jun 05, 2023 at 10:54:16AM +0200, Domenico Cerasuolo wrote:
> @@ -884,14 +842,6 @@ static inline bool obj_allocated(struct page *page, void *obj, unsigned long *ph
>  	return obj_tagged(page, obj, phandle, OBJ_ALLOCATED_TAG);
>  }
>  
> -#ifdef CONFIG_ZPOOL
> -static bool obj_stores_deferred_handle(struct page *page, void *obj,
> -		unsigned long *phandle)
> -{
> -	return obj_tagged(page, obj, phandle, OBJ_DEFERRED_HANDLE_TAG);
> -}
> -#endif

You can actually remove even more here.

The entire concept of deferred_handle is about serializing free with
reclaim. It can all go: OBJ_DEFERRED_HANDLE_TAG, the member in struct
link_free, this function here, find_deferred_handle_obj() (declaration
and implementation), free_handles(), and the deferred handle bits in
obj_free() including the handle parameter itself.
  

On Mon, Jun 5, 2023 at 8:37 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Mon, Jun 05, 2023 at 10:54:16AM +0200, Domenico Cerasuolo wrote:
> > @@ -884,14 +842,6 @@ static inline bool obj_allocated(struct page *page, void *obj, unsigned long *ph
> >       return obj_tagged(page, obj, phandle, OBJ_ALLOCATED_TAG);
> >  }
> >
> > -#ifdef CONFIG_ZPOOL
> > -static bool obj_stores_deferred_handle(struct page *page, void *obj,
> > -             unsigned long *phandle)
> > -{
> > -     return obj_tagged(page, obj, phandle, OBJ_DEFERRED_HANDLE_TAG);
> > -}
> > -#endif
>
> You can actually remove even more here.
>
> The entire concept of deferred_handle is about serializing free with
> reclaim. It can all go: OBJ_DEFERRED_HANDLE_TAG, the member in struct
> link_free, this function here, find_deferred_handle_obj() (declaration
> and implementation), free_handles(), and the deferred handle bits in
> obj_free() including the handle parameter itself.

For more context on this:
https://lore.kernel.org/all/20230110231701.326724-1-nphamcs@gmail.com/T/#u
  

On Mon, Jun 5, 2023 at 1:54 AM Domenico Cerasuolo
<cerasuolodomenico@gmail.com> wrote:
>
> With the recent enhancement to zswap enabling direct page writeback, the
> need for the shrink code in zsmalloc has become obsolete. As a result,
> this commit removes the page reclaim logic from zsmalloc entirely.
>
> Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
> ---
>  mm/zsmalloc.c | 291 +-------------------------------------------------
>  1 file changed, 2 insertions(+), 289 deletions(-)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 02f7f414aade..c87a60514f21 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -250,13 +250,6 @@ struct zs_pool {
>         /* Compact classes */
>         struct shrinker shrinker;
>
> -#ifdef CONFIG_ZPOOL
> -       /* List tracking the zspages in LRU order by most recently added object */
> -       struct list_head lru;
> -       struct zpool *zpool;
> -       const struct zpool_ops *zpool_ops;
> -#endif
> -
>  #ifdef CONFIG_ZSMALLOC_STAT
>         struct dentry *stat_dentry;
>  #endif
> @@ -279,13 +272,6 @@ struct zspage {
>         unsigned int freeobj;
>         struct page *first_page;
>         struct list_head list; /* fullness list */
> -
> -#ifdef CONFIG_ZPOOL
> -       /* links the zspage to the lru list in the pool */
> -       struct list_head lru;
> -       bool under_reclaim;
> -#endif
> -
>         struct zs_pool *pool;
>         rwlock_t lock;
>  };
> @@ -393,14 +379,7 @@ static void *zs_zpool_create(const char *name, gfp_t gfp,
>          * different contexts and its caller must provide a valid
>          * gfp mask.
>          */
> -       struct zs_pool *pool = zs_create_pool(name);
> -
> -       if (pool) {
> -               pool->zpool = zpool;
> -               pool->zpool_ops = zpool_ops;
> -       }
> -
> -       return pool;
> +       return zs_create_pool(name);
>  }
>
>  static void zs_zpool_destroy(void *pool)
> @@ -422,27 +401,6 @@ static void zs_zpool_free(void *pool, unsigned long handle)
>         zs_free(pool, handle);
>  }
>
> -static int zs_reclaim_page(struct zs_pool *pool, unsigned int retries);
> -
> -static int zs_zpool_shrink(void *pool, unsigned int pages,
> -                       unsigned int *reclaimed)
> -{
> -       unsigned int total = 0;
> -       int ret = -EINVAL;
> -
> -       while (total < pages) {
> -               ret = zs_reclaim_page(pool, 8);
> -               if (ret < 0)
> -                       break;
> -               total++;
> -       }
> -
> -       if (reclaimed)
> -               *reclaimed = total;
> -
> -       return ret;
> -}
> -
>  static void *zs_zpool_map(void *pool, unsigned long handle,
>                         enum zpool_mapmode mm)
>  {
> @@ -481,7 +439,7 @@ static struct zpool_driver zs_zpool_driver = {
>         .malloc_support_movable = true,
>         .malloc =                 zs_zpool_malloc,
>         .free =                   zs_zpool_free,
> -       .shrink =                 zs_zpool_shrink,
> +       .shrink =                 NULL,
>         .map =                    zs_zpool_map,
>         .unmap =                  zs_zpool_unmap,
>         .total_size =             zs_zpool_total_size,
> @@ -884,14 +842,6 @@ static inline bool obj_allocated(struct page *page, void *obj, unsigned long *ph
>         return obj_tagged(page, obj, phandle, OBJ_ALLOCATED_TAG);
>  }
>
> -#ifdef CONFIG_ZPOOL
> -static bool obj_stores_deferred_handle(struct page *page, void *obj,
> -               unsigned long *phandle)
> -{
> -       return obj_tagged(page, obj, phandle, OBJ_DEFERRED_HANDLE_TAG);
> -}
> -#endif
> -
>  static void reset_page(struct page *page)
>  {
>         __ClearPageMovable(page);
> @@ -1006,9 +956,6 @@ static void free_zspage(struct zs_pool *pool, struct size_class *class,
>         }
>
>         remove_zspage(class, zspage, ZS_INUSE_RATIO_0);
> -#ifdef CONFIG_ZPOOL
> -       list_del(&zspage->lru);
> -#endif
>         __free_zspage(pool, class, zspage);
>  }
>
> @@ -1054,11 +1001,6 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
>                 off %= PAGE_SIZE;
>         }
>
> -#ifdef CONFIG_ZPOOL
> -       INIT_LIST_HEAD(&zspage->lru);
> -       zspage->under_reclaim = false;
> -#endif
> -
>         set_freeobj(zspage, 0);
>  }
>
> @@ -1525,13 +1467,6 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
>         /* We completely set up zspage so mark them as movable */
>         SetZsPageMovable(pool, zspage);
>  out:
> -#ifdef CONFIG_ZPOOL
> -       /* Add/move zspage to beginning of LRU */
> -       if (!list_empty(&zspage->lru))
> -               list_del(&zspage->lru);
> -       list_add(&zspage->lru, &pool->lru);
> -#endif
> -
>         spin_unlock(&pool->lock);
>
>         return handle;
> @@ -1600,20 +1535,6 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
>         class = zspage_class(pool, zspage);
>
>         class_stat_dec(class, ZS_OBJS_INUSE, 1);
> -
> -#ifdef CONFIG_ZPOOL
> -       if (zspage->under_reclaim) {
> -               /*
> -                * Reclaim needs the handles during writeback. It'll free
> -                * them along with the zspage when it's done with them.
> -                *
> -                * Record current deferred handle in the object's header.
> -                */
> -               obj_free(class->size, obj, &handle);
> -               spin_unlock(&pool->lock);
> -               return;
> -       }
> -#endif
>         obj_free(class->size, obj, NULL);
>
>         fullness = fix_fullness_group(class, zspage);
> @@ -1890,23 +1811,6 @@ static void lock_zspage(struct zspage *zspage)
>  }
>  #endif /* defined(CONFIG_ZPOOL) || defined(CONFIG_COMPACTION) */

If I recall correctly, the defined(CONFIG_ZPOOL) condition is
only needed for the lock_zspage() call in zs_reclaim_page().

You might be able to get away with just
#ifdef CONFIG_COMPACTION if you're removing writeback.

Do fact-check me of course - try to build with these CONFIGs turned
off and see. I'm surprised kernel test robot has not complained about
unused static function lock_zspage() in the case
CONFIG_ZPOOL && !CONFIG_COMPACTION

>
> -#ifdef CONFIG_ZPOOL
> -/*
> - * Unlocks all the pages of the zspage.
> - *
> - * pool->lock must be held before this function is called
> - * to prevent the underlying pages from migrating.
> - */
> -static void unlock_zspage(struct zspage *zspage)
> -{
> -       struct page *page = get_first_page(zspage);
> -
> -       do {
> -               unlock_page(page);
> -       } while ((page = get_next_page(page)) != NULL);
> -}
> -#endif /* CONFIG_ZPOOL */
> -
>  static void migrate_lock_init(struct zspage *zspage)
>  {
>         rwlock_init(&zspage->lock);
> @@ -2126,9 +2030,6 @@ static void async_free_zspage(struct work_struct *work)
>                 VM_BUG_ON(fullness != ZS_INUSE_RATIO_0);
>                 class = pool->size_class[class_idx];
>                 spin_lock(&pool->lock);
> -#ifdef CONFIG_ZPOOL
> -               list_del(&zspage->lru);
> -#endif
>                 __free_zspage(pool, class, zspage);
>                 spin_unlock(&pool->lock);
>         }
> @@ -2474,10 +2375,6 @@ struct zs_pool *zs_create_pool(const char *name)
>          */
>         zs_register_shrinker(pool);
>
> -#ifdef CONFIG_ZPOOL
> -       INIT_LIST_HEAD(&pool->lru);
> -#endif
> -
>         return pool;
>
>  err:
> @@ -2520,190 +2417,6 @@ void zs_destroy_pool(struct zs_pool *pool)
>  }
>  EXPORT_SYMBOL_GPL(zs_destroy_pool);
>
> -#ifdef CONFIG_ZPOOL
> -static void restore_freelist(struct zs_pool *pool, struct size_class *class,
> -               struct zspage *zspage)
> -{
> -       unsigned int obj_idx = 0;
> -       unsigned long handle, off = 0; /* off is within-page offset */
> -       struct page *page = get_first_page(zspage);
> -       struct link_free *prev_free = NULL;
> -       void *prev_page_vaddr = NULL;
> -
> -       /* in case no free object found */
> -       set_freeobj(zspage, (unsigned int)(-1UL));
> -
> -       while (page) {
> -               void *vaddr = kmap_atomic(page);
> -               struct page *next_page;
> -
> -               while (off < PAGE_SIZE) {
> -                       void *obj_addr = vaddr + off;
> -
> -                       /* skip allocated object */
> -                       if (obj_allocated(page, obj_addr, &handle)) {
> -                               obj_idx++;
> -                               off += class->size;
> -                               continue;
> -                       }
> -
> -                       /* free deferred handle from reclaim attempt */
> -                       if (obj_stores_deferred_handle(page, obj_addr, &handle))
> -                               cache_free_handle(pool, handle);
> -
> -                       if (prev_free)
> -                               prev_free->next = obj_idx << OBJ_TAG_BITS;
> -                       else /* first free object found */
> -                               set_freeobj(zspage, obj_idx);
> -
> -                       prev_free = (struct link_free *)vaddr + off / sizeof(*prev_free);
> -                       /* if last free object in a previous page, need to unmap */
> -                       if (prev_page_vaddr) {
> -                               kunmap_atomic(prev_page_vaddr);
> -                               prev_page_vaddr = NULL;
> -                       }
> -
> -                       obj_idx++;
> -                       off += class->size;
> -               }
> -
> -               /*
> -                * Handle the last (full or partial) object on this page.
> -                */
> -               next_page = get_next_page(page);
> -               if (next_page) {
> -                       if (!prev_free || prev_page_vaddr) {
> -                               /*
> -                                * There is no free object in this page, so we can safely
> -                                * unmap it.
> -                                */
> -                               kunmap_atomic(vaddr);
> -                       } else {
> -                               /* update prev_page_vaddr since prev_free is on this page */
> -                               prev_page_vaddr = vaddr;
> -                       }
> -               } else { /* this is the last page */
> -                       if (prev_free) {
> -                               /*
> -                                * Reset OBJ_TAG_BITS bit to last link to tell
> -                                * whether it's allocated object or not.
> -                                */
> -                               prev_free->next = -1UL << OBJ_TAG_BITS;
> -                       }
> -
> -                       /* unmap previous page (if not done yet) */
> -                       if (prev_page_vaddr) {
> -                               kunmap_atomic(prev_page_vaddr);
> -                               prev_page_vaddr = NULL;
> -                       }
> -
> -                       kunmap_atomic(vaddr);
> -               }
> -
> -               page = next_page;
> -               off %= PAGE_SIZE;
> -       }
> -}
> -
> -static int zs_reclaim_page(struct zs_pool *pool, unsigned int retries)
> -{
> -       int i, obj_idx, ret = 0;
> -       unsigned long handle;
> -       struct zspage *zspage;
> -       struct page *page;
> -       int fullness;
> -
> -       /* Lock LRU and fullness list */
> -       spin_lock(&pool->lock);
> -       if (list_empty(&pool->lru)) {
> -               spin_unlock(&pool->lock);
> -               return -EINVAL;
> -       }
> -
> -       for (i = 0; i < retries; i++) {
> -               struct size_class *class;
> -
> -               zspage = list_last_entry(&pool->lru, struct zspage, lru);
> -               list_del(&zspage->lru);
> -
> -               /* zs_free may free objects, but not the zspage and handles */
> -               zspage->under_reclaim = true;
> -
> -               class = zspage_class(pool, zspage);
> -               fullness = get_fullness_group(class, zspage);
> -
> -               /* Lock out object allocations and object compaction */
> -               remove_zspage(class, zspage, fullness);
> -
> -               spin_unlock(&pool->lock);
> -               cond_resched();
> -
> -               /* Lock backing pages into place */
> -               lock_zspage(zspage);
> -
> -               obj_idx = 0;
> -               page = get_first_page(zspage);
> -               while (1) {
> -                       handle = find_alloced_obj(class, page, &obj_idx);
> -                       if (!handle) {
> -                               page = get_next_page(page);
> -                               if (!page)
> -                                       break;
> -                               obj_idx = 0;
> -                               continue;
> -                       }
> -
> -                       /*
> -                        * This will write the object and call zs_free.
> -                        *
> -                        * zs_free will free the object, but the
> -                        * under_reclaim flag prevents it from freeing
> -                        * the zspage altogether. This is necessary so
> -                        * that we can continue working with the
> -                        * zspage potentially after the last object
> -                        * has been freed.
> -                        */
> -                       ret = pool->zpool_ops->evict(pool->zpool, handle);
> -                       if (ret)
> -                               goto next;
> -
> -                       obj_idx++;
> -               }
> -
> -next:
> -               /* For freeing the zspage, or putting it back in the pool and LRU list. */
> -               spin_lock(&pool->lock);
> -               zspage->under_reclaim = false;
> -
> -               if (!get_zspage_inuse(zspage)) {
> -                       /*
> -                        * Fullness went stale as zs_free() won't touch it
> -                        * while the page is removed from the pool. Fix it
> -                        * up for the check in __free_zspage().
> -                        */
> -                       zspage->fullness = ZS_INUSE_RATIO_0;
> -
> -                       __free_zspage(pool, class, zspage);
> -                       spin_unlock(&pool->lock);
> -                       return 0;
> -               }
> -
> -               /*
> -                * Eviction fails on one of the handles, so we need to restore zspage.
> -                * We need to rebuild its freelist (and free stored deferred handles),
> -                * put it back to the correct size class, and add it to the LRU list.
> -                */
> -               restore_freelist(pool, class, zspage);
> -               putback_zspage(class, zspage);
> -               list_add(&zspage->lru, &pool->lru);
> -               unlock_zspage(zspage);
> -       }
> -
> -       spin_unlock(&pool->lock);
> -       return -EAGAIN;
> -}
> -#endif /* CONFIG_ZPOOL */
> -
>  static int __init zs_init(void)
>  {
>         int ret;
> --
> 2.34.1
>
  

On Tue, Jun 6, 2023 at 1:37 AM Nhat Pham <nphamcs@gmail.com> wrote:
>
> On Mon, Jun 5, 2023 at 1:54 AM Domenico Cerasuolo
> <cerasuolodomenico@gmail.com> wrote:
> >
> > With the recent enhancement to zswap enabling direct page writeback, the
> > need for the shrink code in zsmalloc has become obsolete. As a result,
> > this commit removes the page reclaim logic from zsmalloc entirely.
> >
> > Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
> > ---
> >  mm/zsmalloc.c | 291 +-------------------------------------------------
> >  1 file changed, 2 insertions(+), 289 deletions(-)
> >
> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > index 02f7f414aade..c87a60514f21 100644
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -250,13 +250,6 @@ struct zs_pool {
> >         /* Compact classes */
> >         struct shrinker shrinker;
> >
> > -#ifdef CONFIG_ZPOOL
> > -       /* List tracking the zspages in LRU order by most recently added object */
> > -       struct list_head lru;
> > -       struct zpool *zpool;
> > -       const struct zpool_ops *zpool_ops;
> > -#endif
> > -
> >  #ifdef CONFIG_ZSMALLOC_STAT
> >         struct dentry *stat_dentry;
> >  #endif
> > @@ -279,13 +272,6 @@ struct zspage {
> >         unsigned int freeobj;
> >         struct page *first_page;
> >         struct list_head list; /* fullness list */
> > -
> > -#ifdef CONFIG_ZPOOL
> > -       /* links the zspage to the lru list in the pool */
> > -       struct list_head lru;
> > -       bool under_reclaim;
> > -#endif
> > -
> >         struct zs_pool *pool;
> >         rwlock_t lock;
> >  };
> > @@ -393,14 +379,7 @@ static void *zs_zpool_create(const char *name, gfp_t gfp,
> >          * different contexts and its caller must provide a valid
> >          * gfp mask.
> >          */
> > -       struct zs_pool *pool = zs_create_pool(name);
> > -
> > -       if (pool) {
> > -               pool->zpool = zpool;
> > -               pool->zpool_ops = zpool_ops;
> > -       }
> > -
> > -       return pool;
> > +       return zs_create_pool(name);
> >  }
> >
> >  static void zs_zpool_destroy(void *pool)
> > @@ -422,27 +401,6 @@ static void zs_zpool_free(void *pool, unsigned long handle)
> >         zs_free(pool, handle);
> >  }
> >
> > -static int zs_reclaim_page(struct zs_pool *pool, unsigned int retries);
> > -
> > -static int zs_zpool_shrink(void *pool, unsigned int pages,
> > -                       unsigned int *reclaimed)
> > -{
> > -       unsigned int total = 0;
> > -       int ret = -EINVAL;
> > -
> > -       while (total < pages) {
> > -               ret = zs_reclaim_page(pool, 8);
> > -               if (ret < 0)
> > -                       break;
> > -               total++;
> > -       }
> > -
> > -       if (reclaimed)
> > -               *reclaimed = total;
> > -
> > -       return ret;
> > -}
> > -
> >  static void *zs_zpool_map(void *pool, unsigned long handle,
> >                         enum zpool_mapmode mm)
> >  {
> > @@ -481,7 +439,7 @@ static struct zpool_driver zs_zpool_driver = {
> >         .malloc_support_movable = true,
> >         .malloc =                 zs_zpool_malloc,
> >         .free =                   zs_zpool_free,
> > -       .shrink =                 zs_zpool_shrink,
> > +       .shrink =                 NULL,
> >         .map =                    zs_zpool_map,
> >         .unmap =                  zs_zpool_unmap,
> >         .total_size =             zs_zpool_total_size,
> > @@ -884,14 +842,6 @@ static inline bool obj_allocated(struct page *page, void *obj, unsigned long *ph
> >         return obj_tagged(page, obj, phandle, OBJ_ALLOCATED_TAG);
> >  }
> >
> > -#ifdef CONFIG_ZPOOL
> > -static bool obj_stores_deferred_handle(struct page *page, void *obj,
> > -               unsigned long *phandle)
> > -{
> > -       return obj_tagged(page, obj, phandle, OBJ_DEFERRED_HANDLE_TAG);
> > -}
> > -#endif
> > -
> >  static void reset_page(struct page *page)
> >  {
> >         __ClearPageMovable(page);
> > @@ -1006,9 +956,6 @@ static void free_zspage(struct zs_pool *pool, struct size_class *class,
> >         }
> >
> >         remove_zspage(class, zspage, ZS_INUSE_RATIO_0);
> > -#ifdef CONFIG_ZPOOL
> > -       list_del(&zspage->lru);
> > -#endif
> >         __free_zspage(pool, class, zspage);
> >  }
> >
> > @@ -1054,11 +1001,6 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
> >                 off %= PAGE_SIZE;
> >         }
> >
> > -#ifdef CONFIG_ZPOOL
> > -       INIT_LIST_HEAD(&zspage->lru);
> > -       zspage->under_reclaim = false;
> > -#endif
> > -
> >         set_freeobj(zspage, 0);
> >  }
> >
> > @@ -1525,13 +1467,6 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
> >         /* We completely set up zspage so mark them as movable */
> >         SetZsPageMovable(pool, zspage);
> >  out:
> > -#ifdef CONFIG_ZPOOL
> > -       /* Add/move zspage to beginning of LRU */
> > -       if (!list_empty(&zspage->lru))
> > -               list_del(&zspage->lru);
> > -       list_add(&zspage->lru, &pool->lru);
> > -#endif
> > -
> >         spin_unlock(&pool->lock);
> >
> >         return handle;
> > @@ -1600,20 +1535,6 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
> >         class = zspage_class(pool, zspage);
> >
> >         class_stat_dec(class, ZS_OBJS_INUSE, 1);
> > -
> > -#ifdef CONFIG_ZPOOL
> > -       if (zspage->under_reclaim) {
> > -               /*
> > -                * Reclaim needs the handles during writeback. It'll free
> > -                * them along with the zspage when it's done with them.
> > -                *
> > -                * Record current deferred handle in the object's header.
> > -                */
> > -               obj_free(class->size, obj, &handle);
> > -               spin_unlock(&pool->lock);
> > -               return;
> > -       }
> > -#endif
> >         obj_free(class->size, obj, NULL);
> >
> >         fullness = fix_fullness_group(class, zspage);
> > @@ -1890,23 +1811,6 @@ static void lock_zspage(struct zspage *zspage)
> >  }
> >  #endif /* defined(CONFIG_ZPOOL) || defined(CONFIG_COMPACTION) */
>
> If I recall correctly, the defined(CONFIG_ZPOOL) condition is
> only needed for the lock_zspage() call in zs_reclaim_page().
>
> You might be able to get away with just
> #ifdef CONFIG_COMPACTION if you're removing writeback.
>
> Do fact-check me of course - try to build with these CONFIGs turned
> off and see. I'm surprised kernel test robot has not complained about
> unused static function lock_zspage() in the case
> CONFIG_ZPOOL && !CONFIG_COMPACTION
>

Thanks! It is indeed the case, with CONFIG_ZPOOL && !CONFIG_COMPACTION
lock_zspage becomes unused -> will go with #ifdef CONFIG_COMPACTION alone.

> >
> > -#ifdef CONFIG_ZPOOL
> > -/*
> > - * Unlocks all the pages of the zspage.
> > - *
> > - * pool->lock must be held before this function is called
> > - * to prevent the underlying pages from migrating.
> > - */
> > -static void unlock_zspage(struct zspage *zspage)
> > -{
> > -       struct page *page = get_first_page(zspage);
> > -
> > -       do {
> > -               unlock_page(page);
> > -       } while ((page = get_next_page(page)) != NULL);
> > -}
> > -#endif /* CONFIG_ZPOOL */
> > -
> >  static void migrate_lock_init(struct zspage *zspage)
> >  {
> >         rwlock_init(&zspage->lock);
> > @@ -2126,9 +2030,6 @@ static void async_free_zspage(struct work_struct *work)
> >                 VM_BUG_ON(fullness != ZS_INUSE_RATIO_0);
> >                 class = pool->size_class[class_idx];
> >                 spin_lock(&pool->lock);
> > -#ifdef CONFIG_ZPOOL
> > -               list_del(&zspage->lru);
> > -#endif
> >                 __free_zspage(pool, class, zspage);
> >                 spin_unlock(&pool->lock);
> >         }
> > @@ -2474,10 +2375,6 @@ struct zs_pool *zs_create_pool(const char *name)
> >          */
> >         zs_register_shrinker(pool);
> >
> > -#ifdef CONFIG_ZPOOL
> > -       INIT_LIST_HEAD(&pool->lru);
> > -#endif
> > -
> >         return pool;
> >
> >  err:
> > @@ -2520,190 +2417,6 @@ void zs_destroy_pool(struct zs_pool *pool)
> >  }
> >  EXPORT_SYMBOL_GPL(zs_destroy_pool);
> >
> > -#ifdef CONFIG_ZPOOL
> > -static void restore_freelist(struct zs_pool *pool, struct size_class *class,
> > -               struct zspage *zspage)
> > -{
> > -       unsigned int obj_idx = 0;
> > -       unsigned long handle, off = 0; /* off is within-page offset */
> > -       struct page *page = get_first_page(zspage);
> > -       struct link_free *prev_free = NULL;
> > -       void *prev_page_vaddr = NULL;
> > -
> > -       /* in case no free object found */
> > -       set_freeobj(zspage, (unsigned int)(-1UL));
> > -
> > -       while (page) {
> > -               void *vaddr = kmap_atomic(page);
> > -               struct page *next_page;
> > -
> > -               while (off < PAGE_SIZE) {
> > -                       void *obj_addr = vaddr + off;
> > -
> > -                       /* skip allocated object */
> > -                       if (obj_allocated(page, obj_addr, &handle)) {
> > -                               obj_idx++;
> > -                               off += class->size;
> > -                               continue;
> > -                       }
> > -
> > -                       /* free deferred handle from reclaim attempt */
> > -                       if (obj_stores_deferred_handle(page, obj_addr, &handle))
> > -                               cache_free_handle(pool, handle);
> > -
> > -                       if (prev_free)
> > -                               prev_free->next = obj_idx << OBJ_TAG_BITS;
> > -                       else /* first free object found */
> > -                               set_freeobj(zspage, obj_idx);
> > -
> > -                       prev_free = (struct link_free *)vaddr + off / sizeof(*prev_free);
> > -                       /* if last free object in a previous page, need to unmap */
> > -                       if (prev_page_vaddr) {
> > -                               kunmap_atomic(prev_page_vaddr);
> > -                               prev_page_vaddr = NULL;
> > -                       }
> > -
> > -                       obj_idx++;
> > -                       off += class->size;
> > -               }
> > -
> > -               /*
> > -                * Handle the last (full or partial) object on this page.
> > -                */
> > -               next_page = get_next_page(page);
> > -               if (next_page) {
> > -                       if (!prev_free || prev_page_vaddr) {
> > -                               /*
> > -                                * There is no free object in this page, so we can safely
> > -                                * unmap it.
> > -                                */
> > -                               kunmap_atomic(vaddr);
> > -                       } else {
> > -                               /* update prev_page_vaddr since prev_free is on this page */
> > -                               prev_page_vaddr = vaddr;
> > -                       }
> > -               } else { /* this is the last page */
> > -                       if (prev_free) {
> > -                               /*
> > -                                * Reset OBJ_TAG_BITS bit to last link to tell
> > -                                * whether it's allocated object or not.
> > -                                */
> > -                               prev_free->next = -1UL << OBJ_TAG_BITS;
> > -                       }
> > -
> > -                       /* unmap previous page (if not done yet) */
> > -                       if (prev_page_vaddr) {
> > -                               kunmap_atomic(prev_page_vaddr);
> > -                               prev_page_vaddr = NULL;
> > -                       }
> > -
> > -                       kunmap_atomic(vaddr);
> > -               }
> > -
> > -               page = next_page;
> > -               off %= PAGE_SIZE;
> > -       }
> > -}
> > -
> > -static int zs_reclaim_page(struct zs_pool *pool, unsigned int retries)
> > -{
> > -       int i, obj_idx, ret = 0;
> > -       unsigned long handle;
> > -       struct zspage *zspage;
> > -       struct page *page;
> > -       int fullness;
> > -
> > -       /* Lock LRU and fullness list */
> > -       spin_lock(&pool->lock);
> > -       if (list_empty(&pool->lru)) {
> > -               spin_unlock(&pool->lock);
> > -               return -EINVAL;
> > -       }
> > -
> > -       for (i = 0; i < retries; i++) {
> > -               struct size_class *class;
> > -
> > -               zspage = list_last_entry(&pool->lru, struct zspage, lru);
> > -               list_del(&zspage->lru);
> > -
> > -               /* zs_free may free objects, but not the zspage and handles */
> > -               zspage->under_reclaim = true;
> > -
> > -               class = zspage_class(pool, zspage);
> > -               fullness = get_fullness_group(class, zspage);
> > -
> > -               /* Lock out object allocations and object compaction */
> > -               remove_zspage(class, zspage, fullness);
> > -
> > -               spin_unlock(&pool->lock);
> > -               cond_resched();
> > -
> > -               /* Lock backing pages into place */
> > -               lock_zspage(zspage);
> > -
> > -               obj_idx = 0;
> > -               page = get_first_page(zspage);
> > -               while (1) {
> > -                       handle = find_alloced_obj(class, page, &obj_idx);
> > -                       if (!handle) {
> > -                               page = get_next_page(page);
> > -                               if (!page)
> > -                                       break;
> > -                               obj_idx = 0;
> > -                               continue;
> > -                       }
> > -
> > -                       /*
> > -                        * This will write the object and call zs_free.
> > -                        *
> > -                        * zs_free will free the object, but the
> > -                        * under_reclaim flag prevents it from freeing
> > -                        * the zspage altogether. This is necessary so
> > -                        * that we can continue working with the
> > -                        * zspage potentially after the last object
> > -                        * has been freed.
> > -                        */
> > -                       ret = pool->zpool_ops->evict(pool->zpool, handle);
> > -                       if (ret)
> > -                               goto next;
> > -
> > -                       obj_idx++;
> > -               }
> > -
> > -next:
> > -               /* For freeing the zspage, or putting it back in the pool and LRU list. */
> > -               spin_lock(&pool->lock);
> > -               zspage->under_reclaim = false;
> > -
> > -               if (!get_zspage_inuse(zspage)) {
> > -                       /*
> > -                        * Fullness went stale as zs_free() won't touch it
> > -                        * while the page is removed from the pool. Fix it
> > -                        * up for the check in __free_zspage().
> > -                        */
> > -                       zspage->fullness = ZS_INUSE_RATIO_0;
> > -
> > -                       __free_zspage(pool, class, zspage);
> > -                       spin_unlock(&pool->lock);
> > -                       return 0;
> > -               }
> > -
> > -               /*
> > -                * Eviction fails on one of the handles, so we need to restore zspage.
> > -                * We need to rebuild its freelist (and free stored deferred handles),
> > -                * put it back to the correct size class, and add it to the LRU list.
> > -                */
> > -               restore_freelist(pool, class, zspage);
> > -               putback_zspage(class, zspage);
> > -               list_add(&zspage->lru, &pool->lru);
> > -               unlock_zspage(zspage);
> > -       }
> > -
> > -       spin_unlock(&pool->lock);
> > -       return -EAGAIN;
> > -}
> > -#endif /* CONFIG_ZPOOL */
> > -
> >  static int __init zs_init(void)
> >  {
> >         int ret;
> > --
> > 2.34.1
> >