mm: userland pgtable freeing is RCU-safe now, remove leftover bits

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 userspace page
tables are 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.

This is all that is guaranteed, however, so naturally it is still
incumbent upon page table walkers to ensure that the page table entries
are as expected.

Link: https://lore.kernel.org/20260925-rcu-pagetable-freeing-v5-11-31e91065fea4@kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Guo Ren <guoren@kernel.org>
Cc: Brian Cain <bcain@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Simon Schuster <schuster.simon@siemens-energy.com>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Cc: Stafford Horne <shorne@gmail.com>
Cc: Rich Felker <dalias@libc.org>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: Paul Walmsley <pjw@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Vineet Gupta <vgupta@kernel.org>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Will Deacon <will@kernel.org>
Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Magnus Lindholm <linmag7@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: WANG Xuerui <kernel@xen0n.name>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: James Bottomley <james.bottomley@hansenpartnership.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Christophe Leroy <chleroy@kernel.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Qi Zheng <qi.zheng@linux.dev>

1 files changed, 9 insertions, 57 deletions

diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index bdcc2778ac64..9d827076db19 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -67,11 +67,8 @@
  *  - tlb_remove_table()
  *
  *    tlb_remove_table() is the basic primitive to free page-table directories
- *    (__p*_free_tlb()).  In it's most primitive form it is an alias for
- *    tlb_remove_page() below, for when page directories are pages and have no
- *    additional constraints.
- *
- *    See also MMU_GATHER_TABLE_FREE and MMU_GATHER_RCU_TABLE_FREE.
+ *    (__p*_free_tlb()).  Page directories are freed after an RCU grace
+ *    period - see the comment in mm/mmu_gather.c.
  *
  *  - tlb_remove_page() / tlb_remove_page_size()
  *  - __tlb_remove_folio_pages() / __tlb_remove_page_size()
@@ -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.
  *
- *  MMU_GATHER_RCU_TABLE_FREE
- *
- *  Like MMU_GATHER_TABLE_FREE, and adds semi-RCU semantics to the free (see
- *  comment below).
- *
- *  Useful if your architecture doesn't use IPIs for remote TLB invalidates
- *  and therefore doesn't naturally serialize with software page-table walkers.
+ *  An architecture is expected to provide __tlb_remove_table() (see
+ *  HAVE_ARCH_TLB_REMOVE_TABLE) or use the generic __tlb_remove_table(), which
+ *  does the actual freeing of these pages.
  *
  *  MMU_GATHER_NO_FLUSH_CACHE
  *
@@ -200,12 +188,8 @@
  *  various ptep_get_and_clear() functions.
  */
 
-#ifdef CONFIG_MMU_GATHER_TABLE_FREE
-
 struct mmu_table_batch {
-#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
 	struct rcu_head		rcu;
-#endif
 	unsigned int		nr;
 	void			*tables[];
 };
@@ -224,23 +208,6 @@ static inline void __tlb_remove_table(void *table)
 
 extern void tlb_remove_table(struct mmu_gather *tlb, void *table);
 
-#else /* !CONFIG_MMU_GATHER_TABLE_FREE */
-
-static inline void tlb_remove_page(struct mmu_gather *tlb, struct page *page);
-/*
- * Without MMU_GATHER_TABLE_FREE the architecture is assumed to have page based
- * page directories and we can use the normal page batching to free them.
- */
-static inline void tlb_remove_table(struct mmu_gather *tlb, void *table)
-{
-	struct ptdesc *ptdesc = (struct ptdesc *)table;
-
-	pagetable_dtor(ptdesc);
-	tlb_remove_page(tlb, ptdesc_page(ptdesc));
-}
-#endif /* CONFIG_MMU_GATHER_TABLE_FREE */
-
-#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
 /*
  * This allows an architecture that does not use the linux page-tables for
  * hardware to skip the TLBI when freeing page tables.
@@ -253,19 +220,6 @@ void tlb_remove_table_sync_one(void);
 
 void tlb_remove_table_sync_rcu(void);
 
-#else
-
-#ifdef tlb_needs_table_invalidate
-#error tlb_needs_table_invalidate() requires MMU_GATHER_RCU_TABLE_FREE
-#endif
-
-static inline void tlb_remove_table_sync_one(void) { }
-
-static inline void tlb_remove_table_sync_rcu(void) { }
-
-#endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */
-
-
 #ifndef CONFIG_MMU_GATHER_NO_GATHER
 /*
  * If we can't allocate a page to make a big batch of page pointers
@@ -325,9 +279,7 @@ static inline void tlb_flush_rmaps(struct mmu_gather *tlb, struct vm_area_struct
 struct mmu_gather {
 	struct mm_struct	*mm;
 
-#ifdef CONFIG_MMU_GATHER_TABLE_FREE
 	struct mmu_table_batch	*batch;
-#endif
 
 	unsigned long		start;
 	unsigned long		end;