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

Yosry Ahmed
2025-10-24T20:05:59Z

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