Re: [RFC PATCH] mm: bypass swap readahead for zswap

[Kairui Song]

On Wed, Jun 24, 2026 at 3:59 PM Alexandre Ghiti <alex@xxxxxxxx> wrote:
>
> Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous
> device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead.
>
> zswap is the same kind of in-memory, synchronous backend as zram, not a
> swap device flagged SWP_SYNCHRONOUS_IO so it still goes through
> swapin_readahead().
>
> Here are the results from bypassing readahead for zswap too: it was
> measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker
> off, on Sapphire Rapids and 3 iterations.
>
>   768M memcg (sustained swap thrash):
>     metric                 mm-new    + bypass    delta
>     build time (s)          405.0       341.7    -15.6%
>     zswap-in (GB)            79.5        53.0     -33%
>     zswap-out (GB)          144.8       115.6     -20%
>     swap readahead (pages)  6.79M       0.45M     -93%
>     swap_ra hit (%)          72.1        89.9     +18pp
>
>   1G memcg (light pressure, build not memory-bound):
>     metric                 mm-new    + bypass    delta
>     build time (s)          177.7       176.0    ~same (no regression)
>     zswap-in (GB)            10.2         7.5     -26%
>     zswap-out (GB)           27.7        25.1      -9%
>     swap readahead (pages)  1.07M       0.08M     -93%
>     swap_ra hit (%)          68.6        87.2     +19pp
>
> The gain is from no longer prefetching pages that are pointless for an
> in-memory backend: readahead inflates anon residency and thrashes the
> page cache (file pages get evicted and re-read), lengthens each fault by
> synchronously (de)compressing a cluster of neighbours, and adds
> compression traffic when those extra pages are reclaimed.
>
> Bypassing swap readahead for zswap therefore makes sense.
>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> ---
>
> - This bypass originally comes from Usama's series that implements
>   large folio zswapin: while working on improving this series, I noticed
>   the gains I got only came from the bypass of readahead.
>
>  include/linux/zswap.h |  6 ++++++
>  mm/memory.c           |  5 +++--
>  mm/zswap.c            | 11 +++++++++++
>  3 files changed, 20 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> index 30c193a1207e..b6f0e6198b6f 100644
> --- a/include/linux/zswap.h
> +++ b/include/linux/zswap.h
> @@ -35,6 +35,7 @@ void zswap_lruvec_state_init(struct lruvec *lruvec);
>  void zswap_folio_swapin(struct folio *folio);
>  bool zswap_is_enabled(void);
>  bool zswap_never_enabled(void);
> +bool zswap_present_test(swp_entry_t swp);
>  #else
>
>  struct zswap_lruvec_state {};
> @@ -69,6 +70,11 @@ static inline bool zswap_never_enabled(void)
>         return true;
>  }
>
> +static inline bool zswap_present_test(swp_entry_t swp)
> +{
> +       return false;
> +}
> +
>  #endif
>
>  #endif /* _LINUX_ZSWAP_H */
> diff --git a/mm/memory.c b/mm/memory.c
> index ff338c2abe92..5aa1ea9eb48a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>         if (folio)
>                 swap_update_readahead(folio, vma, vmf->address);
>         if (!folio) {
> -               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */
> -               if (data_race(si->flags & SWP_SYNCHRONOUS_IO))
> +               /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */
> +               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
> +                   zswap_present_test(entry))

Hi Alexandre

Thanks for the test and patch, very interesting idea.

> diff --git a/mm/zswap.c b/mm/zswap.c
> index 761cd699e0a3..5b85b4d17647 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -234,6 +234,17 @@ static inline struct xarray *swap_zswap_tree(swp_entry_t swp)
>                 >> ZSWAP_ADDRESS_SPACE_SHIFT];
>  }
>
> +/**
> + * zswap_present_test - check if a swap entry is currently backed by zswap
> + * @swp: the swap entry to test
> + *
> + * Return: true if @swp has a zswap entry, false otherwise.
> + */
> +bool zswap_present_test(swp_entry_t swp)
> +{
> +       return xa_load(swap_zswap_tree(swp), swp_offset(swp));

Better check zswap_never_enabled first to avoid a xa_load if not needed.