linux-kernel - Re: [PATCH v3 3/6] mm: introduce pte_move_swp_offset() helper which can move offset bidirectionally

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Date: Fri, 3 May 2024 10:41:26 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org
Cc: baolin.wang@...ux.alibaba.com, chrisl@...nel.org, david@...hat.com,
 hanchuanhua@...o.com, hannes@...xchg.org, hughd@...gle.com,
 kasong@...cent.com, linux-kernel@...r.kernel.org, surenb@...gle.com,
 v-songbaohua@...o.com, willy@...radead.org, xiang@...nel.org,
 ying.huang@...el.com, yosryahmed@...gle.com, yuzhao@...gle.com,
 ziy@...dia.com
Subject: Re: [PATCH v3 3/6] mm: introduce pte_move_swp_offset() helper which
 can move offset bidirectionally

On 03/05/2024 01:50, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> There could arise a necessity to obtain the first pte_t from a swap
> pte_t located in the middle. For instance, this may occur within the
> context of do_swap_page(), where a page fault can potentially occur in
> any PTE of a large folio. To address this, the following patch introduces
> pte_move_swp_offset(), a function capable of bidirectional movement by
> a specified delta argument. Consequently, pte_increment_swp_offset()

You mean pte_next_swp_offset()?

> will directly invoke it with delta = 1.
> 
> Suggested-by: "Huang, Ying" <ying.huang@...el.com>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
>  mm/internal.h | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index c5552d35d995..cfe4aed66a5c 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -211,18 +211,21 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>  }
>  
>  /**
> - * pte_next_swp_offset - Increment the swap entry offset field of a swap pte.
> + * pte_move_swp_offset - Move the swap entry offset field of a swap pte
> + *	 forward or backward by delta
>   * @pte: The initial pte state; is_swap_pte(pte) must be true and
>   *	 non_swap_entry() must be false.
> + * @delta: The direction and the offset we are moving; forward if delta
> + *	 is positive; backward if delta is negative
>   *
> - * Increments the swap offset, while maintaining all other fields, including
> + * Moves the swap offset, while maintaining all other fields, including
>   * swap type, and any swp pte bits. The resulting pte is returned.
>   */
> -static inline pte_t pte_next_swp_offset(pte_t pte)
> +static inline pte_t pte_move_swp_offset(pte_t pte, long delta)

We have equivalent functions for pfn:

  pte_next_pfn()
  pte_advance_pfn()

Although the latter takes an unsigned long and only moves forward currently. I
wonder if it makes sense to have their naming and semantics match? i.e. change
pte_advance_pfn() to pte_move_pfn() and let it move backwards too.

I guess we don't have a need for that and it adds more churn.

Anyway:

Reviewed-by: Ryan Roberts <ryan.roberts@....com>


>  {
>  	swp_entry_t entry = pte_to_swp_entry(pte);
>  	pte_t new = __swp_entry_to_pte(__swp_entry(swp_type(entry),
> -						   (swp_offset(entry) + 1)));
> +						   (swp_offset(entry) + delta)));
>  
>  	if (pte_swp_soft_dirty(pte))
>  		new = pte_swp_mksoft_dirty(new);
> @@ -234,6 +237,20 @@ static inline pte_t pte_next_swp_offset(pte_t pte)
>  	return new;
>  }
>  
> +
> +/**
> + * pte_next_swp_offset - Increment the swap entry offset field of a swap pte.
> + * @pte: The initial pte state; is_swap_pte(pte) must be true and
> + *	 non_swap_entry() must be false.
> + *
> + * Increments the swap offset, while maintaining all other fields, including
> + * swap type, and any swp pte bits. The resulting pte is returned.
> + */
> +static inline pte_t pte_next_swp_offset(pte_t pte)
> +{
> +	return pte_move_swp_offset(pte, 1);
> +}
> +
>  /**
>   * swap_pte_batch - detect a PTE batch for a set of contiguous swap entries
>   * @start_ptep: Page table pointer for the first entry.