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

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Message-ID: <20250624124042.45beda75@thinkpad-T15>
Date: Tue, 24 Jun 2025 12:40:42 +0200
From: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual
 <anshuman.khandual@....com>,
        Matthew Wilcox <willy@...radead.org>,
        LKML
 <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        linux-s390@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] mm/debug_vm_pgtable: Use a swp_entry_t input
 value for swap tests

On Mon, 23 Jun 2025 21:10:54 +0200
David Hildenbrand <david@...hat.com> wrote:

> 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.

Right, it already felt awkward when I wrote it, not sure why I only changed
it for "swapped entry (arch)". I think I like "swap PTE/PMD", naming the actual
entries in the page table, vs. "swap entry (arch)", naming the (arch-dependent)
representation of the swap PTE/PMD as swp_entry_t.

Will change, and also adjust my description, where I also used possibly
misleading "swapped".

> 
> >   +---------------------------+--------------------------------------------------+
> > -| __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.
> 

Thanks!