Re: [External] [PATCH v2 4/6] riscv: pgtable: unconditionally use tlb_remove_ptdesc()

Hi Qi,

On Tue, Feb 25, 2025 at 11:48 AM Qi Zheng <[email protected]> wrote:
>
> To support fast gup, the commit 69be3fb111e7 ("riscv: enable
> MMU_GATHER_RCU_TABLE_FREE for SMP && MMU") did the following:
>
> 1) use tlb_remove_page_ptdesc() for those platforms which use IPI to
>    perform TLB shootdown
>
> 2) use tlb_remove_ptdesc() for those platforms which use SBI to perform
>    TLB shootdown
>
> The tlb_remove_page_ptdesc() is the wrapper of the tlb_remove_page(). By
> design, the tlb_remove_page() should be used to remove a normal page from
> a page table entry, and should not be used for page table pages.
>
> The tlb_remove_ptdesc() is the wrapper of the tlb_remove_table(), which is
> designed specifically for freeing page table pages. If the
> CONFIG_MMU_GATHER_TABLE_FREE is enabled, the tlb_remove_table() will use
> semi RCU to free page table pages, that is:
>
>  - batch table freeing: asynchronous free by RCU
>  - single table freeing: IPI + synchronous free
>
> If the CONFIG_MMU_GATHER_TABLE_FREE is disabled, the tlb_remove_table()
> will fall back to pagetable_dtor() + tlb_remove_page().
>
> For case 1), since we need to perform TLB shootdown before freeing the
> page table page, the local_irq_save() in fast gup can block the freeing
> and protect the fast gup page walker. Therefore we can ensure safety by
> just using tlb_remove_page_ptdesc(). In addition, we can also the
> tlb_remove_ptdesc()/tlb_remove_table() to achieve it, and it doesn't
> matter whether CONFIG_MMU_GATHER_RCU_TABLE_FREE is selected. And in
> theory, the performance of freeing pages asynchronously via RCU will not
> be lower than synchronous free.
>
> For case 2), since local_irq_save() only disable S-privilege IPI irq but
> not M-privilege's, which is used by the SBI implementation to perform TLB
> shootdown, so we must select CONFIG_MMU_GATHER_RCU_TABLE_FREE and use
> tlb_remove_ptdesc() to ensure safety. The riscv selects this config for
> SMP && MMU, the CONFIG_RISCV_SBI is dependent on MMU. Therefore, only the
> UP system may have the situation where CONFIG_MMU_GATHER_RCU_TABLE_FREE is
> disabled but CONFIG_RISCV_SBI is enabled. But there is no freeing vs fast
> gup race in the UP system.
>
> So, in summary, we can use tlb_remove_ptdesc() to support fast gup in all
> cases, and this interface is specifically designed for page table pages.
> So let's use it unconditionally.
>
> Signed-off-by: Qi Zheng <[email protected]>
> Suggested-by: Peter Zijlstra (Intel) <[email protected]>
> ---
>  arch/riscv/include/asm/pgalloc.h | 26 ++++----------------------
>  1 file changed, 4 insertions(+), 22 deletions(-)
>
> diff --git a/arch/riscv/include/asm/pgalloc.h b/arch/riscv/include/asm/pgalloc.h
> index 3e2aebea6312d..770ce18a7328b 100644
> --- a/arch/riscv/include/asm/pgalloc.h
> +++ b/arch/riscv/include/asm/pgalloc.h
> @@ -15,24 +15,6 @@
>  #define __HAVE_ARCH_PUD_FREE
>  #include <asm-generic/pgalloc.h>
>
> -/*
> - * While riscv platforms with riscv_ipi_for_rfence as true require an IPI to
> - * perform TLB shootdown, some platforms with riscv_ipi_for_rfence as false use
> - * SBI to perform TLB shootdown. 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.
> - */
> -static inline void riscv_tlb_remove_ptdesc(struct mmu_gather *tlb, void *pt)
> -{
> -       if (riscv_use_sbi_for_rfence()) {
> -               tlb_remove_ptdesc(tlb, pt);
> -       } else {
> -               pagetable_dtor(pt);
> -               tlb_remove_page_ptdesc(tlb, pt);
> -       }
> -}
> -
>  static inline void pmd_populate_kernel(struct mm_struct *mm,
>         pmd_t *pmd, pte_t *pte)
>  {
> @@ -108,14 +90,14 @@ static inline void __pud_free_tlb(struct mmu_gather *tlb, pud_t *pud,
>                                   unsigned long addr)
>  {
>         if (pgtable_l4_enabled)
> -               riscv_tlb_remove_ptdesc(tlb, virt_to_ptdesc(pud));
> +               tlb_remove_ptdesc(tlb, virt_to_ptdesc(pud));
>  }
>
>  static inline void __p4d_free_tlb(struct mmu_gather *tlb, p4d_t *p4d,
>                                   unsigned long addr)
>  {
>         if (pgtable_l5_enabled)
> -               riscv_tlb_remove_ptdesc(tlb, virt_to_ptdesc(p4d));
> +               tlb_remove_ptdesc(tlb, virt_to_ptdesc(p4d));
>  }
>  #endif /* __PAGETABLE_PMD_FOLDED */
>
> @@ -143,7 +125,7 @@ static inline pgd_t *pgd_alloc(struct mm_struct *mm)
>  static inline void __pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmd,
>                                   unsigned long addr)
>  {
> -       riscv_tlb_remove_ptdesc(tlb, virt_to_ptdesc(pmd));
> +       tlb_remove_ptdesc(tlb, virt_to_ptdesc(pmd));
>  }
>
>  #endif /* __PAGETABLE_PMD_FOLDED */
> @@ -151,7 +133,7 @@ static inline void __pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmd,
>  static inline void __pte_free_tlb(struct mmu_gather *tlb, pgtable_t pte,
>                                   unsigned long addr)
>  {
> -       riscv_tlb_remove_ptdesc(tlb, page_ptdesc(pte));
> +       tlb_remove_ptdesc(tlb, page_ptdesc(pte));
>  }
>  #endif /* CONFIG_MMU */
>
> --
> 2.20.1
>
>

This set of patches mainly refactors the remove page table function
interfaces. The comment "riscv_ipi_for_rfence" has been removed, as it
was no longer needed, and this patch handles that.

Additionally, whether riscv_use_sbi_for_rfence is true or false, page
tables can now be released using RCU. This patch changes the previous
synchronous release logic (for !riscv_use_sbi_for_rfence) to an
RCU-based release.

So, Reviewed-by: Yunhui Cui <[email protected]>

Based on qemu-system-riscv64, I tested this patch. The log is as follows:
./gup_test
TAP version 13
1..1
# GUP_FAST_BENCHMARK: Time: get:663365 put:117 us
# ok 1 ioctl status 0
# Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0

So, Tested-by: Yunhui Cui <[email protected]>

Thanks,
Yunhui