mm: remove arch_flush_lazy_mmu_mode()

mm: remove arch_flush_lazy_mmu_mode()

Patch series "Nesting support for lazy MMU mode", v2.

When the lazy MMU mode was introduced eons ago, it wasn't made clear
whether such a sequence was legal:

	arch_enter_lazy_mmu_mode()
	...
		arch_enter_lazy_mmu_mode()
		...
		arch_leave_lazy_mmu_mode()
	...
	arch_leave_lazy_mmu_mode()

It seems fair to say that nested calls to
arch_{enter,leave}_lazy_mmu_mode() were not expected, and most
architectures never explicitly supported it.

Ryan Roberts' series from March [1] attempted to prevent nesting from ever
occurring, and mostly succeeded.  Unfortunately, a corner case
(DEBUG_PAGEALLOC) may still cause nesting to occur on arm64.  Ryan
proposed [2] to address that corner case at the generic level but this
approach received pushback; [3] then attempted to solve the issue on arm64
only, but it was deemed too fragile.

It feels generally fragile to rely on lazy_mmu sections not to nest,
because callers of various standard mm functions do not know if the
function uses lazy_mmu itself.  This series therefore performs a U-turn
and adds support for nested lazy_mmu sections, on all architectures.

The main change enabling nesting is patch 2, following the approach
suggested by Catalin Marinas [4]: have enter() return some state and the
matching leave() take that state.  In this series, the state is only used
to handle nesting, but it could be used for other purposes such as
restoring context modified by enter(); the proposed kpkeys framework would
be an immediate user [5].

This patch (of 7):

This function has only ever been used in arch/x86, so there is no need for
other architectures to implement it.  Remove it from linux/pgtable.h and
all architectures besides x86.

The arm64 implementation is not empty but it is only called from
arch_leave_lazy_mmu_mode(), so we can simply fold it there.

Link: https://lkml.kernel.org/r/20250908073931.4159362-1-kevin.brodsky@arm.com
Link: https://lkml.kernel.org/r/20250908073931.4159362-2-kevin.brodsky@arm.com
Link: https://lore.kernel.org/all/20250303141542.3371656-1-ryan.roberts@arm.com/
Link: https://lore.kernel.org/all/20250530140446.2387131-1-ryan.roberts@arm.com/
Link: https://lore.kernel.org/all/20250606135654.178300-1-ryan.roberts@arm.com/
Link: https://lore.kernel.org/all/aEhKSq0zVaUJkomX@arm.com/
Link: https://lore.kernel.org/linux-hardening/20250815085512.2182322-19-kevin.brodsky@arm.com/
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: David S. Miller <davem@davemloft.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Juegren Gross <jgross@suse.com>
Cc: levi.yun <yeoreum.yun@arm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>