linux-kernel - Re: [PATCH v7 3/8] mm: zswap: Refactor code to store an entry in zswap xarray.

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Message-ID: <CAJD7tkZbx3q5soiwu1V9pVNCy35QDXK_SFQa6cr19-kXZgU8xw@mail.gmail.com>
Date: Tue, 24 Sep 2024 12:14:59 -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 v7 3/8] mm: zswap: Refactor code to store an entry in
 zswap xarray.

On Mon, Sep 23, 2024 at 6:17 PM Kanchana P Sridhar
<kanchana.p.sridhar@...el.com> wrote:
>
> Added a new procedure zswap_store_entry() that refactors the code
> currently in zswap_store() to store an entry in the zswap xarray.
> This will allow us to call this procedure for each storing the swap
> offset of each page in an mTHP in the xarray, as part of zswap_store()
> supporting mTHP.
>
> Also, made a minor edit in the comments for 'struct zswap_entry' to delete
> the description of the 'value' member that was deleted in commit
> 20a5532ffa53d6ecf41ded920a7b0ff9c65a7dcf ("mm: remove code to handle
> same filled pages").
>
> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
> ---
>  mm/zswap.c | 51 ++++++++++++++++++++++++++++++++++-----------------
>  1 file changed, 34 insertions(+), 17 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 59b7733a62d3..fd35a81b6e36 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -190,7 +190,6 @@ static struct shrinker *zswap_shrinker;
>   *              section for context.
>   * pool - the zswap_pool the entry's data is in
>   * handle - zpool allocation handle that stores the compressed page data
> - * value - value of the same-value filled pages which have same content
>   * objcg - the obj_cgroup that the compressed memory is charged to
>   * lru - handle to the pool's lru used to evict pages.
>   */
> @@ -1404,12 +1403,44 @@ static void shrink_worker(struct work_struct *w)
>  /*********************************
>  * main API
>  **********************************/
> +
> +/*
> + * Returns true if the entry was successfully
> + * stored in the xarray, and false otherwise.
> + */
> +static bool zswap_store_entry(struct xarray *tree,
> +                             struct zswap_entry *entry)


I think zswap_tree_store() is a more descriptive name.

>
> +{
> +       struct zswap_entry *old;
> +       pgoff_t offset = swp_offset(entry->swpentry);


Reverse xmas tree where possible please (longest to shortest declarations).

>
> +
> +       old = xa_store(tree, offset, entry, GFP_KERNEL);
> +

No need for the blank line here.

> +       if (xa_is_err(old)) {
> +               int err = xa_err(old);
> +
> +               WARN_ONCE(err != -ENOMEM, "unexpected xarray error: %d\n", err);
> +               zswap_reject_alloc_fail++;
> +               return false;
> +       }
> +
> +       /*
> +        * We may have had an existing entry that became stale when
> +        * the folio was redirtied and now the new version is being
> +        * swapped out. Get rid of the old.
> +        */
> +       if (old)
> +               zswap_entry_free(old);
> +
> +       return true;
> +}
> +
>  bool zswap_store(struct folio *folio)
>  {
>         swp_entry_t swp = folio->swap;
>         pgoff_t offset = swp_offset(swp);
>         struct xarray *tree = swap_zswap_tree(swp);
> -       struct zswap_entry *entry, *old;
> +       struct zswap_entry *entry;
>         struct obj_cgroup *objcg = NULL;
>         struct mem_cgroup *memcg = NULL;
>
> @@ -1465,22 +1496,8 @@ bool zswap_store(struct folio *folio)
>         entry->objcg = objcg;
>         entry->referenced = true;
>
> -       old = xa_store(tree, offset, entry, GFP_KERNEL);
> -       if (xa_is_err(old)) {
> -               int err = xa_err(old);
> -
> -               WARN_ONCE(err != -ENOMEM, "unexpected xarray error: %d\n", err);
> -               zswap_reject_alloc_fail++;
> +       if (!zswap_store_entry(tree, entry))
>                 goto store_failed;
> -       }
> -
> -       /*
> -        * We may have had an existing entry that became stale when
> -        * the folio was redirtied and now the new version is being
> -        * swapped out. Get rid of the old.
> -        */
> -       if (old)
> -               zswap_entry_free(old);
>
>         if (objcg) {
>                 obj_cgroup_charge_zswap(objcg, entry->length);
> --
> 2.27.0
>