Re: [PATCH 00/16] mm: remove is_swap_[pte, pmd]() + non-swap entries, introduce leaf entries

[Wei Yang]

On Mon, Nov 03, 2025 at 12:31:41PM +0000, 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 series therefore introduces the concept of 'leaf entries' to eliminate
>this confusion.
>
>A leaf entry in this sense is any page table entry which is non-present,
>and represented by the leaf_entry_t type.
>
>This includes 'none' or empty entries, which are simply represented by an
>zero leaf entry value.
>
>In order to maintain compatibility as we transition the kernel to this new
>type, we simply typedef swp_entry_t to leaf_entry_t.
>
>We introduce a number of predicates and helpers to interact with leaf
>entries in include/linux/leafops.h which, as it imports swapops.h, can be
>treated as a drop-in replacement for swapops.h wherever leaf entry helpers
>are used.
>
>Since leafent_from_[pte, pmd]() treats present entries as they were
>empty/none leaf entries, this allows for a great deal of simplification of
>code throughout the code base, which this series utilises a great deal.
>
>We additionally change from swap entry to leaf entry handling where it
>makes sense to and eliminate functions from swapops.h where leaf entries
>obviate the need for the functions.
>

Hi, Lorenzo

Thanks for the effort on cleanup this, which helps me clearing the confusing
on checking swap entry.


-- 
Wei Yang
Help you, Help me