Re: [PATCH v2 11/12] mm: make userland page table freeing RCU-safe
From: Kiryl Shutsemau <kas@kernel.org>
Date: 2026-09-09 09:15:39
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On Tue, Sep 08, 2026 at 01:32:20PM +0100, Lorenzo Stoakes (ARM) wrote:
Now every architecture has been converted to support
CONFIG_MMU_GATHER_RCU_TABLE_FREE, this configuration option no longer makes
any sense to keep around.
Therefore remove it, and remove all the dead code that existed for
!CONFIG_MMU_GATHER_RCU_TABLE_FREE architectures previously.
Additionally, CONFIG_MMU_GATHER_TABLE_FREE is no longer necessary, as all
architectures instead use CONFIG_HAVE_ARCH_TLB_REMOVE_TABLE when a custom
__tlb_remove_table() is required, so remove this too.
A number of architectures only enabled CONFIG_MMU_GATHER_RCU_TABLE_FREE if
CONFIG_MMU was set, however the mmu_gather logic only actually does
something meaningful if CONFIG_MMU is set (mmu_gather.c is only compiled in
this case, for instance).
As a result, there's no need to gate any of this logic on CONFIG_MMU
explicitly.
CONFIG_PT_RECLAIM however does have a strict dependency on CONFIG_MMU, so
make this dependency explicit.
Additionally, correct comments to remove references to non-RCU page table
gathering and make it clear that this is not 'semi-RCU', nor has it been
since commit 1fb3d8c20bfa ("mm/mmu_gather: replace IPI with
synchronize_rcu() when batch allocation fails").
With this change in place the kernel policy is now that all page tables areYou missed s/all/userspace/ from v1.
freed after an RCU grace period, and thus it is now safe to unconditionally perform page table walks under RCU, safe in the knowledge that page tables will not be freed underneath the walker.
quoted hunk ↗ jump to hunk
@@ -151,24 +148,15 @@ * This might be useful if your architecture has size specific TLB * invalidation instructions. * - * MMU_GATHER_TABLE_FREE - * - * This provides tlb_remove_table(), to be used instead of tlb_remove_page() - * for page directores (__p*_free_tlb()). - * - * Useful if your architecture has non-page page directories. + * Page directories (__p*_free_tlb()) are always freed via tlb_remove_table(), + * after an RCU grace period (see mm/mmu_gather.c). * - * When used, an architecture is expected to provide __tlb_remove_table() or - * use the generic __tlb_remove_table(), which does the actual freeing of these - * pages. + * This serialises against software page-table walkers, including architectures + * which do not use IPIs for remote TLB invalidates.
Indentation is broken. -- Kiryl Shutsemau / Kirill A. Shutemov