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

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@kernel.org>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---

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