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References: <20230605085419.44383-1-cerasuolodomenico@gmail.com> <20230605085419.44383-5-cerasuolodomenico@gmail.com>
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From:   Nhat Pham <nphamcs@gmail.com>
Date:   Mon, 5 Jun 2023 16:37:46 -0700
Message-ID: <CAKEwX=OsG5YNEeAVgOeNfiWewahUrUP6ftvwdMo1eENxBi0qCg@mail.gmail.com>
Subject: Re: [RFC PATCH 4/7] mm: zswap: remove page reclaim logic from zsmalloc
To:     Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Cc:     vitaly.wool@konsulko.com, minchan@kernel.org,
        senozhatsky@chromium.org, yosryahmed@google.com,
        linux-mm@kvack.org, ddstreet@ieee.org, sjenning@redhat.com,
        hannes@cmpxchg.org, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, kernel-team@meta.com
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On Mon, Jun 5, 2023 at 1:54=E2=80=AFAM 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 =3D zs_create_pool(name);
> -
> -       if (pool) {
> -               pool->zpool =3D zpool;
> -               pool->zpool_ops =3D 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 =3D 0;
> -       int ret =3D -EINVAL;
> -
> -       while (total < pages) {
> -               ret =3D zs_reclaim_page(pool, 8);
> -               if (ret < 0)
> -                       break;
> -               total++;
> -       }
> -
> -       if (reclaimed)
> -               *reclaimed =3D 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 =3D {
>         .malloc_support_movable =3D true,
>         .malloc =3D                 zs_zpool_malloc,
>         .free =3D                   zs_zpool_free,
> -       .shrink =3D                 zs_zpool_shrink,
> +       .shrink =3D                 NULL,
>         .map =3D                    zs_zpool_map,
>         .unmap =3D                  zs_zpool_unmap,
>         .total_size =3D             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 %=3D PAGE_SIZE;
>         }
>
> -#ifdef CONFIG_ZPOOL
> -       INIT_LIST_HEAD(&zspage->lru);
> -       zspage->under_reclaim =3D 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 h=
andle)
>         class =3D 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 =3D 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 =3D get_first_page(zspage);
> -
> -       do {
> -               unlock_page(page);
> -       } while ((page =3D get_next_page(page)) !=3D 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 *w=
ork)
>                 VM_BUG_ON(fullness !=3D ZS_INUSE_RATIO_0);
>                 class =3D 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 *cl=
ass,
> -               struct zspage *zspage)
> -{
> -       unsigned int obj_idx =3D 0;
> -       unsigned long handle, off =3D 0; /* off is within-page offset */
> -       struct page *page =3D get_first_page(zspage);
> -       struct link_free *prev_free =3D NULL;
> -       void *prev_page_vaddr =3D NULL;
> -
> -       /* in case no free object found */
> -       set_freeobj(zspage, (unsigned int)(-1UL));
> -
> -       while (page) {
> -               void *vaddr =3D kmap_atomic(page);
> -               struct page *next_page;
> -
> -               while (off < PAGE_SIZE) {
> -                       void *obj_addr =3D vaddr + off;
> -
> -                       /* skip allocated object */
> -                       if (obj_allocated(page, obj_addr, &handle)) {
> -                               obj_idx++;
> -                               off +=3D class->size;
> -                               continue;
> -                       }
> -
> -                       /* free deferred handle from reclaim attempt */
> -                       if (obj_stores_deferred_handle(page, obj_addr, &h=
andle))
> -                               cache_free_handle(pool, handle);
> -
> -                       if (prev_free)
> -                               prev_free->next =3D obj_idx << OBJ_TAG_BI=
TS;
> -                       else /* first free object found */
> -                               set_freeobj(zspage, obj_idx);
> -
> -                       prev_free =3D (struct link_free *)vaddr + off / s=
izeof(*prev_free);
> -                       /* if last free object in a previous page, need t=
o unmap */
> -                       if (prev_page_vaddr) {
> -                               kunmap_atomic(prev_page_vaddr);
> -                               prev_page_vaddr =3D NULL;
> -                       }
> -
> -                       obj_idx++;
> -                       off +=3D class->size;
> -               }
> -
> -               /*
> -                * Handle the last (full or partial) object on this page.
> -                */
> -               next_page =3D 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 =3D 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 =3D -1UL << OBJ_TAG_BITS;
> -                       }
> -
> -                       /* unmap previous page (if not done yet) */
> -                       if (prev_page_vaddr) {
> -                               kunmap_atomic(prev_page_vaddr);
> -                               prev_page_vaddr =3D NULL;
> -                       }
> -
> -                       kunmap_atomic(vaddr);
> -               }
> -
> -               page =3D next_page;
> -               off %=3D PAGE_SIZE;
> -       }
> -}
> -
> -static int zs_reclaim_page(struct zs_pool *pool, unsigned int retries)
> -{
> -       int i, obj_idx, ret =3D 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 =3D 0; i < retries; i++) {
> -               struct size_class *class;
> -
> -               zspage =3D list_last_entry(&pool->lru, struct zspage, lru=
);
> -               list_del(&zspage->lru);
> -
> -               /* zs_free may free objects, but not the zspage and handl=
es */
> -               zspage->under_reclaim =3D true;
> -
> -               class =3D zspage_class(pool, zspage);
> -               fullness =3D 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 =3D 0;
> -               page =3D get_first_page(zspage);
> -               while (1) {
> -                       handle =3D find_alloced_obj(class, page, &obj_idx=
);
> -                       if (!handle) {
> -                               page =3D get_next_page(page);
> -                               if (!page)
> -                                       break;
> -                               obj_idx =3D 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 =3D pool->zpool_ops->evict(pool->zpool, handl=
e);
> -                       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 =3D false;
> -
> -               if (!get_zspage_inuse(zspage)) {
> -                       /*
> -                        * Fullness went stale as zs_free() won't touch i=
t
> -                        * while the page is removed from the pool. Fix i=
t
> -                        * up for the check in __free_zspage().
> -                        */
> -                       zspage->fullness =3D 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 re=
store zspage.
> -                * We need to rebuild its freelist (and free stored defer=
red handles),
> -                * put it back to the correct size class, and add it to t=
he 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
>