In order to implement fast gup we need to ensure that the page
table walker is protected from page table pages being freed from
under it.

riscv situation is more complicated than other architectures: some
riscv platforms may use IPI to perform TLB shootdown, for example,
those platforms which support AIA, usually the riscv_ipi_for_rfence is
true on these platforms; some riscv platforms may rely on the SBI to
perform TLB shootdown, usually the riscv_ipi_for_rfence is false on
these platforms. To keep software pagetable walkers safe in this case
we switch to RCU based table free (MMU_GATHER_RCU_TABLE_FREE). See the
comment below 'ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE' in
include/asm-generic/tlb.h for more details.

This patch enables MMU_GATHER_RCU_TABLE_FREE, then use

*tlb_remove_page_ptdesc() for those platforms which use IPI to perform
TLB shootdown;

*tlb_remove_ptdesc() for those platforms which use SBI to perform TLB
shootdown;

Both case mean that disabling interrupts will block the free and
protect the fast gup page walker.
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20231219175046.2496-4-jszhang@kernel.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>