Re: [PATCH v3 2/3] mm: zswap: add zswap_never_enabled()

[Barry Song <21cnbao@gmail.com>]

On Tue, Jun 11, 2024 at 2:45 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> Add zswap_never_enabled() to skip the xarray lookup in zswap_load() if
> zswap was never enabled on the system. It is implemented using static
> branches for efficiency, as enabling zswap should be a rare event. This
> could shave some cycles off zswap_load() when CONFIG_ZSWAP is used but
> zswap is never enabled.
>
> However, the real motivation behind this patch is two-fold:
> - Incoming large folio swapin work will need to fallback to order-0
>   folios if zswap was ever enabled, because any part of the folio could
>   be in zswap, until proper handling of large folios with zswap is
>   added.
>
> - A warning and recovery attempt will be added in a following change in
>   case the above was not done incorrectly. Zswap will fail the read if
>   the folio is large and it was ever enabled.
>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
>  mm/zswap.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index a8c8dd8cfe6f5..7fcd751e847d6 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -83,6 +83,7 @@ static bool zswap_pool_reached_full;
>  static int zswap_setup(void);
>
>  /* Enable/disable zswap */
> +static DEFINE_STATIC_KEY_MAYBE(CONFIG_ZSWAP_DEFAULT_ON, zswap_ever_enabled);
>  static bool zswap_enabled = IS_ENABLED(CONFIG_ZSWAP_DEFAULT_ON);
>  static int zswap_enabled_param_set(const char *,
>                                    const struct kernel_param *);
> @@ -136,6 +137,11 @@ bool zswap_is_enabled(void)
>         return zswap_enabled;
>  }
>
> +static bool zswap_never_enabled(void)
> +{
> +       return !static_branch_maybe(CONFIG_ZSWAP_DEFAULT_ON, &zswap_ever_enabled);
> +}

Will we "extern" this one so that mm-core can use it to fallback
to small folios?
or you prefer this to be done within the coming swapin series?

> +
>  /*********************************
>  * data structures
>  **********************************/
> @@ -1557,6 +1563,9 @@ bool zswap_load(struct folio *folio)
>
>         VM_WARN_ON_ONCE(!folio_test_locked(folio));
>
> +       if (zswap_never_enabled())
> +               return false;
> +
>         /*
>          * When reading into the swapcache, invalidate our entry. The
>          * swapcache can be the authoritative owner of the page and
> @@ -1735,6 +1744,7 @@ static int zswap_setup(void)
>                         zpool_get_type(pool->zpools[0]));
>                 list_add(&pool->list, &zswap_pools);
>                 zswap_has_pool = true;
> +               static_branch_enable(&zswap_ever_enabled);
>         } else {
>                 pr_err("pool creation failed\n");
>                 zswap_enabled = false;
> --
> 2.45.2.505.gda0bf45e8d-goog
>

Thanks
Barry