Re: [PATCH v2 02/19] mm, swap: split swap cache preparation loop into a standalone helper

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

On Mon, Nov 17, 2025 at 2:12 AM Kairui Song <ryncsn@gmail.com> wrote:
>
> From: Kairui Song <kasong@tencent.com>
>
> To prepare for the removal of swap cache bypass swapin, introduce a new
> helper that accepts an allocated and charged fresh folio, prepares the
> folio, the swap map, and then adds the folio to the swap cache.
>
> This doesn't change how swap cache works yet, we are still depending on
> the SWAP_HAS_CACHE in the swap map for synchronization. But all
> synchronization hacks are now all in this single helper.
>
> No feature change.
>
> Signed-off-by: Kairui Song <kasong@tencent.com>

Overall,
Reviewed-by: Barry Song <baohua@kernel.org>
with a few minor nits.

> ---
>  mm/swap_state.c | 197 +++++++++++++++++++++++++++++++-------------------------
>  1 file changed, 109 insertions(+), 88 deletions(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 08252eaef32f..7b93704fcbe7 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -402,6 +402,97 @@ void swap_update_readahead(struct folio *folio, struct vm_area_struct *vma,
>         }
>  }
>
> +/**
> + * __swap_cache_prepare_and_add - Prepare the folio and add it to swap cache.
> + * @entry: swap entry to be bound to the folio.
> + * @folio: folio to be added.
> + * @gfp: memory allocation flags for charge, can be 0 if @charged if true.
> + * @charged: if the folio is already charged.
> + * @skip_if_exists: if the slot is in a cached state, return NULL.
> + *                  This is an old workaround that will be removed shortly.
> + *
> + * Update the swap_map and add folio as swap cache, typically before swapin.
> + * All swap slots covered by the folio must have a non-zero swap count.
> + *
> + * Context: Caller must protect the swap device with reference count or locks.
> + * Return: Returns the folio being added on success. Returns the existing
> + * folio if @entry is cached. Returns NULL if raced with swapin or swapoff.

Not a native speaker, but I wonder if adding *already* would make the
comment clearer, for example: "Returns the existing folio if @entry is
already cached."

BTW, returning NULL may not be the best option. Returning an ERR_PTR
seems more specific?

Thanks
Barry