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References: <20240611024516.1375191-1-yosryahmed@google.com> <20240611024516.1375191-2-yosryahmed@google.com>
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From: Barry Song <21cnbao@gmail.com>
Date: Wed, 12 Jun 2024 09:53:45 +1200
Message-ID: <CAGsJ_4w3LDE1OuDiX_LAeTxEGUFPVOwqMxoOF+Dr55bdLUZQ7w@mail.gmail.com>
Subject: Re: [PATCH v3 2/3] mm: zswap: add zswap_never_enabled()
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Johannes Weiner <hannes@cmpxchg.org>, 
	Nhat Pham <nphamcs@gmail.com>, Chengming Zhou <chengming.zhou@linux.dev>, 
	Chris Li <chrisl@kernel.org>, David Hildenbrand <david@redhat.com>, 
	Matthew Wilcox <willy@infradead.org>, linux-mm@kvack.org, linux-kernel@vger.kernel.org
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On Tue, Jun 11, 2024 at 2:45=E2=80=AFPM 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_enabl=
ed);
>  static bool zswap_enabled =3D 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 =3D true;
> +               static_branch_enable(&zswap_ever_enabled);
>         } else {
>                 pr_err("pool creation failed\n");
>                 zswap_enabled =3D false;
> --
> 2.45.2.505.gda0bf45e8d-goog
>

Thanks
Barry