Re: [RFC PATCH 00/12] remove is_swap_[pte, pmd]() + non-swap confusion

[Yosry Ahmed <yosry.ahmed@linux.dev>]

On Fri, Oct 24, 2025 at 08:41:16AM +0100, Lorenzo Stoakes wrote:
> There's an established convention in the kernel that we treat leaf page
> tables (so far at the PTE, PMD level) as containing 'swap entries' should
> they be neither empty (i.e. p**_none() evaluating true) nor present
> (i.e. p**_present() evaluating true).
> 
> However, at the same time we also have helper predicates - is_swap_pte(),
> is_swap_pmd() - which are inconsistently used.
> 
> This is problematic, as it is logical to assume that should somebody wish
> to operate upon a page table swap entry they should first check to see if
> it is in fact one.
> 
> It also implies that perhaps, in future, we might introduce a non-present,
> none page table entry that is not a swap entry.
> 
> This series resolves this issue by systematically eliminating all use of
> the is_swap_pte() and is swap_pmd() predicates so we retain only the
> convention that should a leaf page table entry be neither none nor present
> it is a swap entry.
> 
> We also have the further issue that 'swap entry' is unfortunately a really
> rather overloaded term and in fact refers to both entries for swap and for
> other information such as migration entries, page table markers, and device
> private entries.
> 
> We therefore have the rather 'unique' concept of a 'non-swap' swap entry.
> 
> This is deeply confusing, so this series goes further and eliminates the
> non_swap_entry() predicate, replacing it with is_non_present_entry() - with
> an eye to a new convention of referring to these non-swap 'swap entries' as
> non-present.

I just wanted to say THANK YOU for doing this. It is indeed a very
annoying and confusing convention, and I wanted to do something about it
in the past but never got around to it..

> 
> It also introduces the is_swap_entry() predicate to explicitly and
> logically refer to actual 'true' swap entries, improving code readibility,
> avoiding the hideous convention of:
> 
> 	if (!non_swap_entry(entry)) {
> 		...
> 	}
> 
> As part of these changes we also introduce a few other new predicates:
> 
> * pte_to_swp_entry_or_zero() - allows for convenient conversion from a PTE
>   to a swap entry if present, or an empty swap entry if none. This is
>   useful as many swap entry conversions are simply checking for flags for
>   which this suffices.
> 
> * get_pte_swap_entry() - Retrieves a PTE swap entry if it truly is a swap
>   entry (i.e. not a non-present entry), returning true if so, otherwise
>   returns false. This simplifies a lot of logic that previously open-coded
>   this.
> 
> * is_huge_pmd() - Determines if a PMD contains either a present transparent
>   huge page entry or a huge non-present entry. This again simplifies a lot
>   of logic that simply open-coded this.
> 
> REVIEWERS NOTE:
> 
> This series applies against mm-unstable as there are currently conflicts
> with mm-new. Should the series receive community assent I will resolve
> these at the point the RFC tag is removed.
> 
> I also intend to use this as a foundation for further work to add higher
> order page table markers.
> 
> Lorenzo Stoakes (12):
>   mm: introduce and use pte_to_swp_entry_or_zero()
>   mm: avoid unnecessary uses of is_swap_pte()
>   mm: introduce get_pte_swap_entry() and use it
>   mm: use get_pte_swap_entry() in debug pgtable + remove is_swap_pte()
>   fs/proc/task_mmu: refactor pagemap_pmd_range()
>   mm: avoid unnecessary use of is_swap_pmd()
>   mm: introduce is_huge_pmd() and use where appropriate
>   mm/huge_memory: refactor copy_huge_pmd() non-present logic
>   mm/huge_memory: refactor change_huge_pmd() non-present logic
>   mm: remove remaining is_swap_pmd() users and is_swap_pmd()
>   mm: rename non_swap_entry() to is_non_present_entry()
>   mm: provide is_swap_entry() and use it
> 
>  arch/s390/mm/gmap_helpers.c   |   2 +-
>  arch/s390/mm/pgtable.c        |   2 +-
>  fs/proc/task_mmu.c            | 214 ++++++++++++++++++++--------------
>  include/linux/huge_mm.h       |  49 +++++---
>  include/linux/swapops.h       |  99 ++++++++++++++--
>  include/linux/userfaultfd_k.h |  16 +--
>  mm/debug_vm_pgtable.c         |  43 ++++---
>  mm/filemap.c                  |   2 +-
>  mm/hmm.c                      |   2 +-
>  mm/huge_memory.c              | 189 ++++++++++++++++--------------
>  mm/hugetlb.c                  |   6 +-
>  mm/internal.h                 |  12 +-
>  mm/khugepaged.c               |  29 ++---
>  mm/madvise.c                  |  14 +--
>  mm/memory.c                   |  62 +++++-----
>  mm/migrate.c                  |   2 +-
>  mm/mincore.c                  |   2 +-
>  mm/mprotect.c                 |  45 ++++---
>  mm/mremap.c                   |   9 +-
>  mm/page_table_check.c         |  25 ++--
>  mm/page_vma_mapped.c          |  30 +++--
>  mm/swap_state.c               |   5 +-
>  mm/swapfile.c                 |   3 +-
>  mm/userfaultfd.c              |   2 +-
>  24 files changed, 511 insertions(+), 353 deletions(-)
> 
> --
> 2.51.0