linux-kernel - Re: [PATCH v8 5/8] mm: zswap: Modify zswap_stored_pages to be atomic_long

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Message-ID: <CAJD7tka0qyRWhgHrC9p1ytfDKVPr9bBTFCYy7HC4DZ-Ovfu7VQ@mail.gmail.com>
Date: Fri, 27 Sep 2024 19:57:49 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, hannes@...xchg.org, 
	nphamcs@...il.com, chengming.zhou@...ux.dev, usamaarif642@...il.com, 
	shakeel.butt@...ux.dev, ryan.roberts@....com, ying.huang@...el.com, 
	21cnbao@...il.com, akpm@...ux-foundation.org, nanhai.zou@...el.com, 
	wajdi.k.feghali@...el.com, vinodh.gopal@...el.com
Subject: Re: [PATCH v8 5/8] mm: zswap: Modify zswap_stored_pages to be atomic_long_t.

On Fri, Sep 27, 2024 at 7:16 PM Kanchana P Sridhar
<kanchana.p.sridhar@...el.com> wrote:
>
> For zswap_store() to support large folios, we need to be able to do
> a batch update of zswap_stored_pages upon successful store of all pages
> in the folio. For this, we need to add folio_nr_pages(), which returns
> a long, to zswap_stored_pages.

Do we really need this? A lot of places in the kernel assign the
result of folio_nr_pages() to an int (thp_nr_pages(),
split_huge_pages_all(), etc). I don't think we need to worry about
folio_nr_pages() exceeding INT_MAX for a while.

>
> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
> ---
>  fs/proc/meminfo.c     |  2 +-
>  include/linux/zswap.h |  2 +-
>  mm/zswap.c            | 19 +++++++++++++------
>  3 files changed, 15 insertions(+), 8 deletions(-)
>
> diff --git a/fs/proc/meminfo.c b/fs/proc/meminfo.c
> index 245171d9164b..8ba9b1472390 100644
> --- a/fs/proc/meminfo.c
> +++ b/fs/proc/meminfo.c
> @@ -91,7 +91,7 @@ static int meminfo_proc_show(struct seq_file *m, void *v)
>  #ifdef CONFIG_ZSWAP
>         show_val_kb(m, "Zswap:          ", zswap_total_pages());
>         seq_printf(m,  "Zswapped:       %8lu kB\n",
> -                  (unsigned long)atomic_read(&zswap_stored_pages) <<
> +                  (unsigned long)atomic_long_read(&zswap_stored_pages) <<
>                    (PAGE_SHIFT - 10));
>  #endif
>         show_val_kb(m, "Dirty:          ",
> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> index 9cd1beef0654..d961ead91bf1 100644
> --- a/include/linux/zswap.h
> +++ b/include/linux/zswap.h
> @@ -7,7 +7,7 @@
>
>  struct lruvec;
>
> -extern atomic_t zswap_stored_pages;
> +extern atomic_long_t zswap_stored_pages;
>
>  #ifdef CONFIG_ZSWAP
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 0f281e50a034..43e4e216db41 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -43,7 +43,7 @@
>  * statistics
>  **********************************/
>  /* The number of compressed pages currently stored in zswap */
> -atomic_t zswap_stored_pages = ATOMIC_INIT(0);
> +atomic_long_t zswap_stored_pages = ATOMIC_INIT(0);
>
>  /*
>   * The statistics below are not protected from concurrent access for
> @@ -802,7 +802,7 @@ static void zswap_entry_free(struct zswap_entry *entry)
>                 obj_cgroup_put(entry->objcg);
>         }
>         zswap_entry_cache_free(entry);
> -       atomic_dec(&zswap_stored_pages);
> +       atomic_long_dec(&zswap_stored_pages);
>  }
>
>  /*********************************
> @@ -1232,7 +1232,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>                 nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
>         } else {
>                 nr_backing = zswap_total_pages();
> -               nr_stored = atomic_read(&zswap_stored_pages);
> +               nr_stored = atomic_long_read(&zswap_stored_pages);
>         }
>
>         if (!nr_stored)
> @@ -1501,7 +1501,7 @@ bool zswap_store(struct folio *folio)
>         }
>
>         /* update stats */
> -       atomic_inc(&zswap_stored_pages);
> +       atomic_long_inc(&zswap_stored_pages);
>         count_vm_event(ZSWPOUT);
>
>         return true;
> @@ -1650,6 +1650,13 @@ static int debugfs_get_total_size(void *data, u64 *val)
>  }
>  DEFINE_DEBUGFS_ATTRIBUTE(total_size_fops, debugfs_get_total_size, NULL, "%llu\n");
>
> +static int debugfs_get_stored_pages(void *data, u64 *val)
> +{
> +       *val = atomic_long_read(&zswap_stored_pages);
> +       return 0;
> +}
> +DEFINE_DEBUGFS_ATTRIBUTE(stored_pages_fops, debugfs_get_stored_pages, NULL, "%llu\n");
> +
>  static int zswap_debugfs_init(void)
>  {
>         if (!debugfs_initialized())
> @@ -1673,8 +1680,8 @@ static int zswap_debugfs_init(void)
>                            zswap_debugfs_root, &zswap_written_back_pages);
>         debugfs_create_file("pool_total_size", 0444,
>                             zswap_debugfs_root, NULL, &total_size_fops);
> -       debugfs_create_atomic_t("stored_pages", 0444,
> -                               zswap_debugfs_root, &zswap_stored_pages);
> +       debugfs_create_file("stored_pages", 0444,
> +                           zswap_debugfs_root, NULL, &stored_pages_fops);
>
>         return 0;
>  }
> --
> 2.27.0
>