linux-kernel - Re: [RFC PATCH 1/1] mm/debug_vm_pgtable: Use a swp_entry

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Message-ID: <9bd91df8-e548-4ecc-bd30-f1ab611ecf4c@redhat.com>
Date: Mon, 23 Jun 2025 21:10:54 +0200
From: David Hildenbrand <david@...hat.com>
To: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Anshuman Khandual <anshuman.khandual@....com>
Cc: Matthew Wilcox <willy@...radead.org>, LKML
 <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC PATCH 1/1] mm/debug_vm_pgtable: Use a swp_entry_t input
 value for swap tests

On 23.06.25 20:43, Gerald Schaefer wrote:
> The various __pte/pmd_to_swp_entry and __swp_entry_to_pte/pmd helper
> functions are expected to operate on swapped PTE/PMD entries, not on
> present and mapped entries.
> 
> Reflect this in the swap tests by using a swp_entry_t as input value,
> similar to how it is already done in pte_swap_exclusive_tests().
> Move the swap entry creation to init_args() and store it in args, so
> it can also be used in other functions.
> 
> The pte/pmd_swap_tests() are also changed to compare entries instead of
> pfn values, because pte/pmd_pfn() helpers are not expected to operate on
> swapped entries. E.g. on s390, pmd_pfn() needs different shifts for leaf
> (large) and non-leaf PMDs.
> 
> Also update documentation, to reflect that the helpers operate on
> swapped and not mapped entries, and use correct names, i.e.
> __swp_to_pte/pmd_entry -> __swp_entry_to_pte/pmd.
> 
> For consistency, also change pte/pmd_swap_soft_dirty_tests() to use
> args->swp_entry instead of a present and mapped PTE/PMD.
> 
> Signed-off-by: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
> ---
>   Documentation/mm/arch_pgtable_helpers.rst |  8 ++--
>   mm/debug_vm_pgtable.c                     | 55 ++++++++++++++---------
>   2 files changed, 38 insertions(+), 25 deletions(-)
> 
> diff --git a/Documentation/mm/arch_pgtable_helpers.rst b/Documentation/mm/arch_pgtable_helpers.rst
> index af245161d8e7..e2ac76202a85 100644
> --- a/Documentation/mm/arch_pgtable_helpers.rst
> +++ b/Documentation/mm/arch_pgtable_helpers.rst
> @@ -242,13 +242,13 @@ SWAP Page Table Helpers
>   ========================
>   
>   +---------------------------+--------------------------------------------------+
> -| __pte_to_swp_entry        | Creates a swapped entry (arch) from a mapped PTE |
> +| __pte_to_swp_entry        | Creates a swap entry (arch) from a swapped PTE   |

Maybe something like:

"from a swap (!none && !present) PTE"

or short

"swap PTE".

"swapped" might be misleading.

Same for the other cases below.

>   +---------------------------+--------------------------------------------------+
> -| __swp_to_pte_entry        | Creates a mapped PTE from a swapped entry (arch) |
> +| __swp_entry_to_pte        | Creates a swapped PTE from a swap entry (arch)   |
>   +---------------------------+--------------------------------------------------+
> -| __pmd_to_swp_entry        | Creates a swapped entry (arch) from a mapped PMD |
> +| __pmd_to_swp_entry        | Creates a swap entry (arch) from a swapped PMD   |
>   +---------------------------+--------------------------------------------------+
> -| __swp_to_pmd_entry        | Creates a mapped PMD from a swapped entry (arch) |
> +| __swp_entry_to_pmd        | Creates a swapped PMD from a swap entry (arch)   |
>   +---------------------------+--------------------------------------------------+
>   | is_migration_entry        | Tests a migration (read or write) swapped entry  |
>   +-------------------------------+----------------------------------------------+
> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> index 7731b238b534..3b0f83ed6c2e 100644
> --- a/mm/debug_vm_pgtable.c
> +++ b/mm/debug_vm_pgtable.c
> @@ -73,6 +73,8 @@ struct pgtable_debug_args {
>   	unsigned long		fixed_pud_pfn;
>   	unsigned long		fixed_pmd_pfn;
>   	unsigned long		fixed_pte_pfn;
> +
> +	swp_entry_t		swp_entry;
>   };
>   

Nothing else jumped at me, so LGTM.

-- 
Cheers,

David / dhildenb