Re: [PATCH v7 4/6] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

[Vitaly Wool]

On Mon, Nov 28, 2022 at 8:16 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
>
> This helps determines the coldest zspages as candidates for writeback.
>
> Signed-off-by: Nhat Pham <nphamcs@xxxxxxxxx>
> ---
>  mm/zsmalloc.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 50 insertions(+)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 5427a00a0518..b1bc231d94a3 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -239,6 +239,11 @@ 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;
> +#endif
> +
>  #ifdef CONFIG_ZSMALLOC_STAT
>         struct dentry *stat_dentry;
>  #endif
> @@ -260,6 +265,12 @@ 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;
> +#endif
> +
>         struct zs_pool *pool;
>  #ifdef CONFIG_COMPACTION
>         rwlock_t lock;
> @@ -953,6 +964,9 @@ static void free_zspage(struct zs_pool *pool, struct size_class *class,
>         }
>
>         remove_zspage(class, zspage, ZS_EMPTY);
> +#ifdef CONFIG_ZPOOL
> +       list_del(&zspage->lru);
> +#endif
>         __free_zspage(pool, class, zspage);
>  }
>
> @@ -998,6 +1012,10 @@ static void init_zspage(struct size_class *class, struct zspage *zspage)
>                 off %= PAGE_SIZE;
>         }
>
> +#ifdef CONFIG_ZPOOL
> +       INIT_LIST_HEAD(&zspage->lru);
> +#endif
> +
>         set_freeobj(zspage, 0);
>  }
>
> @@ -1270,6 +1288,31 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
>         obj_to_location(obj, &page, &obj_idx);
>         zspage = get_zspage(page);
>
> +#ifdef CONFIG_ZPOOL
> +       /*
> +        * Move the zspage to front of pool's LRU.
> +        *
> +        * Note that this is swap-specific, so by definition there are no ongoing
> +        * accesses to the memory while the page is swapped out that would make
> +        * it "hot". A new entry is hot, then ages to the tail until it gets either
> +        * written back or swaps back in.
> +        *
> +        * Furthermore, map is also called during writeback. We must not put an
> +        * isolated page on the LRU mid-reclaim.
> +        *
> +        * As a result, only update the LRU when the page is mapped for write
> +        * when it's first instantiated.
> +        *
> +        * This is a deviation from the other backends, which perform this update
> +        * in the allocation function (zbud_alloc, z3fold_alloc).
> +        */
> +       if (mm == ZS_MM_WO) {
> +               if (!list_empty(&zspage->lru))
> +                       list_del(&zspage->lru);
> +               list_add(&zspage->lru, &pool->lru);
> +       }
> +#endif
> +
>         /*
>          * migration cannot move any zpages in this zspage. Here, pool->lock
>          * is too heavy since callers would take some time until they calls
> @@ -1988,6 +2031,9 @@ static void async_free_zspage(struct work_struct *work)
>                 VM_BUG_ON(fullness != ZS_EMPTY);
>                 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);
>         }
> @@ -2299,6 +2345,10 @@ struct zs_pool *zs_create_pool(const char *name)
>          */
>         zs_register_shrinker(pool);
>
> +#ifdef CONFIG_ZPOOL
> +       INIT_LIST_HEAD(&pool->lru);
> +#endif

I think the amount of #ifdefs here becomes absolutely overwhelming.
Not that zsmalloc code was very readable before, but now it is
starting to look like a plain disaster.

Thanks,
Vitaly

> +
>         return pool;
>
>  err:
> --
> 2.30.2