linux-kernel - Re: [PATCH v3 02/16] mm: introduce leaf entry type and use to simplify leaf entry logic

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Message-ID: <4d8510ae-1ed8-440d-b864-d75147c05510@lucifer.local>
Date: Thu, 13 Nov 2025 15:32:55 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
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Subject: Re: [PATCH v3 02/16] mm: introduce leaf entry type and use to
 simplify leaf entry logic

OK Andrew please disregard and drop this, I was being too paranoid...

Comment at SWAP_ENTRY_INVALID indicates 0,0 is reserved as I thought before.

Cheers, Lorenzo

On Thu, Nov 13, 2025 at 02:56:09PM +0000, Lorenzo Stoakes wrote:
> Hi Andrew,
>
> Please apply the attached fix-patch.
>
> This ensures that we do not accidentally conflict with any valid swap
> entry. We can do so without occupying any additional swap (softleaf) type.
>
> Cheers, Lorenzo
>
> ----8<----
> From 78439310eded5db10692c3e8d0d322bdd6409eff Mon Sep 17 00:00:00 2001
> From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Date: Thu, 13 Nov 2025 14:20:34 +0000
> Subject: [PATCH] mm: avoid any possible collision between swap entries,
>  SOFTLEAF_NONE
>
> The way swap entries are encoded varies by architecture. For x86-64 for
> instance, the encoded swap offset is the one's complement of the specified
> swap offset.
>
> As a result, device 0, offset 0 would be encoded as 0..01..10b.
>
> This means it is possible to specify a PTE entry that is both device 0,
> offset 0 and something that will be identified as a swap entry rather than
> a pte_none() entry.
>
> For other architectures, the encoding may preclude such entries being
> valid.
>
> The softleaf implementation currently depends on a 0..0b entry being
> uniquely identifiable as a none entry.
>
> This is therefore not a safe assumption, so let's fix that.
>
> PTE markers unconditionally occupy a softleaf type, and currently use only
> 3 bits of the offset field to encode their type with no further information
> recorded.
>
> It is therefore no issue at all to add an additional marker type
> designating the field as a none entry.
>
> We also make the none checks more canonical by adjusting softleaf_is_none()
> to reference softleaf_mk_none().
>
> By doing so we avoid any possible collision with swap file entries while
> taking up no further meaningful resource.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
>  include/linux/leafops.h | 8 +++++---
>  include/linux/swap.h    | 1 +
>  include/linux/swapops.h | 6 +++++-
>  3 files changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/leafops.h b/include/linux/leafops.h
> index cff9d94fd5d1..74fd95b55e9c 100644
> --- a/include/linux/leafops.h
> +++ b/include/linux/leafops.h
> @@ -40,7 +40,8 @@ enum softleaf_type {
>   */
>  static inline softleaf_t softleaf_mk_none(void)
>  {
> -	return ((softleaf_t) { 0 });
> +	/* Uniquely identifies none entry. */
> +	return make_pte_marker_entry(PTE_MARKER_SOFTLEAF_NONE);
>  }
>
>  /**
> @@ -72,7 +73,7 @@ static inline softleaf_t softleaf_from_pte(pte_t pte)
>   */
>  static inline bool softleaf_is_none(softleaf_t entry)
>  {
> -	return entry.val == 0;
> +	return entry.val == softleaf_mk_none().val;
>  }
>
>  /**
> @@ -199,7 +200,8 @@ static inline bool softleaf_is_hwpoison(softleaf_t entry)
>   */
>  static inline bool softleaf_is_marker(softleaf_t entry)
>  {
> -	return softleaf_type(entry) == SOFTLEAF_MARKER;
> +	return softleaf_type(entry) == SOFTLEAF_MARKER &&
> +		!softleaf_is_none(entry);
>  }
>
>  /**
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 38ca3df68716..e5abea55448b 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -112,6 +112,7 @@ static inline int current_is_kswapd(void)
>  #define SWP_HWPOISON_NUM 0
>  #endif
>
> +/* Leave a type reserved for softleaf none. */
>  #define MAX_SWAPFILES \
>  	((1 << MAX_SWAPFILES_SHIFT) - SWP_DEVICE_NUM - \
>  	SWP_MIGRATION_NUM - SWP_HWPOISON_NUM - \
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 0a4b3f51ecf5..04e74716a845 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -419,7 +419,11 @@ typedef unsigned long pte_marker;
>   * PROT_NONE, rather than if they were a memory hole or equivalent.
>   */
>  #define  PTE_MARKER_GUARD			BIT(2)
> -#define  PTE_MARKER_MASK			(BIT(3) - 1)
> +
> +/* Internal use by the softleaf implementation to represent 'none' entries. */
> +#define  PTE_MARKER_SOFTLEAF_NONE		BIT(3)
> +
> +#define  PTE_MARKER_MASK			(BIT(4) - 1)
>
>  static inline swp_entry_t make_pte_marker_entry(pte_marker marker)
>  {
> --
> 2.51.0