Re: [RFC 1/4] mm/zswap: skip swapcache for swapping in zswap pages

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, Oct 18, 2024 at 3:50 AM Usama Arif <usamaarif642@gmail.com> wrote:
>
> As mentioned in [1], there is a significant improvement in no
> readahead swapin performance for super fast devices when skipping
> swapcache.
>
> With large folio zswapin support added in later patches, this will also
> mean this path will also act as "readahead" by swapping in multiple
> pages into large folios. further improving performance.
>
> [1] https://lore.kernel.org/all/1505886205-9671-5-git-send-email-minchan@kernel.org/T/#m5a792a04dfea20eb7af4c355d00503efe1c86a93
>
> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> ---
>  include/linux/zswap.h |  6 ++++++
>  mm/memory.c           |  3 ++-
>  mm/page_io.c          |  1 -
>  mm/zswap.c            | 46 +++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 54 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/zswap.h b/include/linux/zswap.h
> index d961ead91bf1..e418d75db738 100644
> --- a/include/linux/zswap.h
> +++ b/include/linux/zswap.h
> @@ -27,6 +27,7 @@ struct zswap_lruvec_state {
>  unsigned long zswap_total_pages(void);
>  bool zswap_store(struct folio *folio);
>  bool zswap_load(struct folio *folio);
> +bool zswap_present_test(swp_entry_t swp, int nr_pages);
>  void zswap_invalidate(swp_entry_t swp);
>  int zswap_swapon(int type, unsigned long nr_pages);
>  void zswap_swapoff(int type);
> @@ -49,6 +50,11 @@ static inline bool zswap_load(struct folio *folio)
>         return false;
>  }
>
> +static inline bool zswap_present_test(swp_entry_t swp, int nr_pages)
> +{
> +       return false;
> +}
> +
>  static inline void zswap_invalidate(swp_entry_t swp) {}
>  static inline int zswap_swapon(int type, unsigned long nr_pages)
>  {
> diff --git a/mm/memory.c b/mm/memory.c
> index 03e5452dd0c0..49d243131169 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4289,7 +4289,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>         swapcache = folio;
>
>         if (!folio) {
> -               if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
> +               if ((data_race(si->flags & SWP_SYNCHRONOUS_IO) ||
> +                   zswap_present_test(entry, 1)) &&
>                     __swap_count(entry) == 1) {
>                         /* skip swapcache */
>                         folio = alloc_swap_folio(vmf);
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 4aa34862676f..2a15b197968a 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -602,7 +602,6 @@ void swap_read_folio(struct folio *folio, struct swap_iocb **plug)
>         unsigned long pflags;
>         bool in_thrashing;
>
> -       VM_BUG_ON_FOLIO(!folio_test_swapcache(folio) && !synchronous, folio);
>         VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
>         VM_BUG_ON_FOLIO(folio_test_uptodate(folio), folio);
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 7f00cc918e7c..f4b03071b2fb 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1576,6 +1576,52 @@ bool zswap_store(struct folio *folio)
>         return ret;
>  }
>
> +static bool swp_offset_in_zswap(unsigned int type, pgoff_t offset)
> +{
> +       return (offset >> SWAP_ADDRESS_SPACE_SHIFT) <  nr_zswap_trees[type];
> +}
> +
> +/* Returns true if the entire folio is in zswap */
> +bool zswap_present_test(swp_entry_t swp, int nr_pages)

Also, did you check how the performance changes if we bring back the
bitmap of present entries (i.e. what used to be frontswap's bitmap)
instead of the tree lookups here?

> +{
> +       pgoff_t offset = swp_offset(swp), tree_max_idx;
> +       int max_idx = 0, i = 0, tree_offset = 0;
> +       unsigned int type = swp_type(swp);
> +       struct zswap_entry *entry = NULL;
> +       struct xarray *tree;
> +
> +       while (i < nr_pages) {
> +               tree_offset = offset + i;
> +               /* Check if the tree exists. */
> +               if (!swp_offset_in_zswap(type, tree_offset))
> +                       return false;
> +
> +               tree = swap_zswap_tree(swp_entry(type, tree_offset));
> +               XA_STATE(xas, tree, tree_offset);
> +
> +               tree_max_idx = tree_offset % SWAP_ADDRESS_SPACE_PAGES ?
> +                       ALIGN(tree_offset, SWAP_ADDRESS_SPACE_PAGES) :
> +                       ALIGN(tree_offset + 1, SWAP_ADDRESS_SPACE_PAGES);
> +               max_idx = min(offset + nr_pages, tree_max_idx) - 1;
> +               rcu_read_lock();
> +               xas_for_each(&xas, entry, max_idx) {
> +                       if (xas_retry(&xas, entry))
> +                               continue;
> +                       i++;
> +               }
> +               rcu_read_unlock();
> +               /*
> +                * If xas_for_each exits because entry is NULL and
> +                * the number of entries checked are less then max idx,
> +                * then zswap does not contain the entire folio.
> +                */
> +               if (!entry && offset + i <= max_idx)
> +                       return false;
> +       }
> +
> +       return true;
> +}
> +
>  bool zswap_load(struct folio *folio)
>  {
>         swp_entry_t swp = folio->swap;
> --
> 2.43.5
>