On 14/07/2026 16:03, Kevin Brodsky wrote:
Page table constructors and destructors were originally introduced
specifically for user PTE pages. Over time, their use became more
widespread, and starting from v6.16 they are called at all levels
and for both user and kernel page tables [1]. Or so I thought. Turns
out, we still have various special kernel page tables where the
ctor/dtor are not called; vmemmap page tables is one such case.
Looking closer, we do not actually need per-level ctors for kernel
page tables. In fact we do not need ctors for them at all, because
nothing is specific to kernel page tables in the ctor/dtor and
nowadays they should be allocated with pagetable_alloc().
Here's the plan: all the generic page table handling moves to
pagetable_{alloc,free}(). Whatever is left is specific to user page
tables; the ctor/dtor are only called for user page tables at the
required levels (PTE/PMD), and renamed to make that clear.
Doing this will allow us to remove a good amount of special casing,
but there's quite a bit of churn to get there. Here's what this
series does:
1. Introduce the concept of "kernel mm" and replace most comparisons
with &init_mm with a flag check (patch 1-12); no functional change
so far.
2. Move the page table accounting/type setting to pagetable_{alloc,free}
(patch 13-15). Kernel mm's do not need to call the ctor/dtor any
more.
The underlying assumption here is that pagetable_{alloc,free} are used
wherever the ctor/dtor are called, and as Sashiko pointed out there are
counterexamples:
1. sparc32 calls the ctor/dtor for user PTE pages, but uses a custom
allocator.
2. archs that do not select MMU_GATHER_TABLE_FREE do not use
pagetable_free() in tlb_remove_table().
3. arch_remove_memory() and vmemmap_free() are a bit of a mess - some
archs call the dtor and/or use pagetable_free() for freeing the direct
map page tables, some don't.
How I'm thinking to address these:
1. sparc32 does not use split ptlocks, so maybe doing the memory
accounting explicitly? Otherwise, we may need to keep the page type
handling + memory accounting in a separate helper that sparc32 can
explicitly call.
2. Always enable MMU_GATHER_TABLE_FREE. pagetable_free() should always
be used for pages allocated with pagetable_alloc().
3. Go through all the implementations of memory hotplug and vmemmap, and
ensure that pagetable_free() is used for those pages allocated with
pagetable_free() (and only those). That may well call for a separate series.
- Kevin