Re: [PATCH v4 08/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for sh-X2

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Wed, Sep 23, 2026 at 07:54:17PM +0800, Lance Yang wrote:
>
> On Tue, Sep 22, 2026 at 04:35:39PM +0100, Lorenzo Stoakes (ARM) wrote:
> >Currently, non-x2 sh specifies CONFIG_MMU_GATHER_RCU_TABLE_FREE allowing
> >RCU page table freeing.
> >
> >sh-X2 is problematic because it utilises slab-allocated PMD page tables,
> >and thus tlb_remove_ptdesc() cannot be used in these cases.
> >
> >All other sh variants are fine as commit e3ecf7c7d082 ("mm: pgtable:
> >convert some architectures to use tlb_remove_ptdesc()") already converted
> >page table freeing to use tlb_remove_ptdesc(), which does so after an RCU
> >grace period when CONFIG_MMU_GATHER_RCU_TABLE_FREE is specified.
> >
> >Resolve this issue by firstly specifying CONFIG_HAVE_ARCH_TLB_REMOVE_TABLE
> >for sh-X2, so the arch can provide its own __tlb_remove_table()
> >implementation (called after the RCU grace period).
> >
> >Then, convert __pmd_free_tlb() to tag the pointer to the PMD, and have
> >__tlb_remove_table() check this tag to determine whether to free via the
> >slab or to use pagetable_dtor_free().
>
> Makes sense to me, so RCU callback won't mistake a slab PMD for a PTE
> page :)

Yes indeed :)

>
> >This follows the pattern used by sparc64 as implemented in commit
> >4a0100f7546f ("sparc64: use RCU page table freeing").
> >
> >Previously __pmd_free_tlb() freed PMD page tables immediately, before any
> >TLB flush IPI. This seems to be a pre-existing bug, which this change also
> >resolves.
> >
> >CONFIG_HAVE_ARCH_TLB_REMOVE_TABLE is only specified for sh-X2, as setting
> >it disables CONFIG_PT_RECLAIM and causes __tlb_remove_table_one() to call
> >tlb_remove_table_sync_rcu() and synchronize_rcu() in turn, and this is not
> >necessary for other sh variants.
> >
> >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>
> >---
>
> LGTM, feel free to add:
>
> Acked-by: Lance Yang <lance.yang@linux.dev>

Thanks!

--
Cheers, Lorenzo