Re: [PATCH v4 10/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for sparc32

[Lance Yang]

On Tue, Sep 22, 2026 at 04:35:41PM +0100, Lorenzo Stoakes (ARM) wrote:
>Careful handling is required for sparc32 which implements page tables as
>part of a shared backing page.
>
>To support this, a custom __tlb_remove_table() function is required, as
>specified by CONFIG_HAVE_ARCH_TLB_REMOVE_TABLE.
>
>This allows __pte_free_tlb() and __pmd_free_tlb() to specify which page
>table level is being freed, which is transmitted to __tlb_remove_table()
>through setting the lowest bit of the page table to 1 for a PMD and 0 for a
>PTE (the page tables are 256-byte aligned so this is safe to do).
>
>Next, since the page table freeing is done via RCU callback, and thus might
>be executed in softirq context, update the spin locks to IRQ save/restore.
>
>Then, in __tlb_remove_table(), figure out whether to free a PMD page table
>via free_pmd_fast() or a PTE via the newly introduced __pte_free()
>function, using the lower bit encoded in __pte_free_tlb() or
>__pmd_free_tlb() to determine which to call.
>
>As part of this change use this spin lock rather than mm->page_table_lock
>for all shared page table exclusion, as RCU freeing means that page tables
>can be freed from soft IRQ context so both don't have an mm and also
>mm->page_table_lock is not IRQ-safe.
>
>Note that the specification of CONFIG_HAVE_ARCH_TLB_REMOVE_TABLE disables
>CONFIG_PT_RECLAIM for sparc32, which mirrors sparc64.
>
>This forms part of an overall effort to switch every architecture to this
>mode, and with it complete, means every architecture now supports
>CONFIG_MMU_GATHER_RCU_TABLE_FREE.
>
>Acked-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
>Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
>---

Also ran page-table teardown stress on qemu-system-sparc (SS-20, sun4m,
CONFIG_SPARC32=y). All 24 rounds ran cleanly :)

Feel free to add:

Tested-by: Lance Yang <lance.yang@xxxxxxxxx>
Acked-by: Lance Yang <lance.yang@xxxxxxxxx>