Re: [PATCH v4 09/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for m68k-motorola

[Lance Yang <lance.yang@linux.dev>]

On Tue, Sep 22, 2026 at 04:35:40PM +0100, Lorenzo Stoakes (ARM) wrote:
>sun3 and coldfire are already supported, however motorola requires a little
>more care.
>
>Here, custom table removal logic is required, so
>CONFIG_HAVE_ARCH_TLB_REMOVE_TABLE is enabled for m68k-motorola.
>
>Firstly as part of this change, the page table level must be communicated
>to the underlying __tlb_remove_table() implementation.
>
>Take advantage of the fact that page tables are aligned by more than enough
>to permit setting TABLE_PTE or TABLE_PMD in the low bits of the pointer,
>and store this there.
>
>Then update __pte_free_tlb() and __pmd_free_tlb() to pass this through,
>then have __tlb_remove_table() decode this and pass it to
>free_pointer_table().
>
>The page table freeing is performed via call_rcu(), so free_pointer_table()
>now will be invoked from softirq context, and as such may be re-entrant.
>
>Introduce an irq save/restore spinlock to handle this, and hold it over the
>time a given ptable entry is being referenced in both get_pointer_table()
>and free_pointer_table().
>
>In order to make things a little easier in this respect, separate out the
>logic for adding a new ptable entry into add_pointer_table() and only hold
>the lock during ptable entry insertion in this case.
>
>Note that original list_add_tail(new, dp) added new prior to dp, which is
>ptable_list[type].next, i.e. after ptable_list[type].
>
>The equivalent therefore is list_add(new, &ptable_list[type]), which adds
>new after ptable_list[type], only without needing to make reference to dp.
>
>Note that, as m68k-motorola specifies CONFIG_HAVE_ARCH_TLB_REMOVE_TABLE, it
>does not enable CONFIG_PT_RECLAIM. This isn't meaningfully impactful.
>
>With this applied, all of m68k implements CONFIG_MMU_GATHER_RCU_TABLE_FREE.
>
>This forms part of an overall effort to switch every architecture to this
>mode.
>
>Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
>Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
>---

I wondered about slot reuse during the RCU wait, but that can't happen :)

The slot stays marked busy until free_pointer_table() runs after RCU, and
the last free unlinks the page under ptable_lock.

Also ran page-table teardown stress and fork/exec churn on
qemu-system-m68k (virt, m68040, CONFIG_MMU_MOTOROLA=y). All 40 rounds ran
cleanly :D

Acked-by: Lance Yang <lance.yang@linux.dev>

Tested-by: Lance Yang <lance.yang@linux.dev>