Re: [PATCH 3/4] riscv: enable MMU_GATHER_RCU_TABLE_FREE for SMP && MMU

[Jisheng Zhang]

On Sun, Dec 31, 2023 at 07:32:47AM +0100, Alexandre Ghiti wrote:
> On 19/12/2023 18:50, Jisheng Zhang wrote:
> > 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;
> 
>
> Can you elaborate a bit more on what those functions do differently and why
> we need to differentiate IPI vs SBI TLB shootdown? I don't understand this.

Hi Alex,

If IPI, the local_irq_save in lockless_pages_from_mm() of fast gup code
path will block page table pages from being freed, I think the comments
there is execellent.

If SBI, the local_irq_save in lockless_pages_from_mm() can't acchieve
the goal however. Because local_irq_save() only disable S-privilege IPI irq,
it can't disable M-privilege's, which the SBI implementation use to
shootdown TLB entry. So we need MMU_GATHER_RCU_TABLE_FREE helper for
SBI case.

Thanks
 
> 
> > Both case mean that disabling interrupts will block the free and
> > protect the fast gup page walker.
> > 
> > Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> > ---
> >   arch/riscv/Kconfig               |  1 +
> >   arch/riscv/include/asm/pgalloc.h | 23 ++++++++++++++++++-----
> >   arch/riscv/include/asm/tlb.h     | 18 ++++++++++++++++++
> >   3 files changed, 37 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 24c1799e2ec4..d3555173d9f4 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -147,6 +147,7 @@ config RISCV
> >   	select IRQ_FORCED_THREADING
> >   	select KASAN_VMALLOC if KASAN
> >   	select LOCK_MM_AND_FIND_VMA
> > +	select MMU_GATHER_RCU_TABLE_FREE if SMP && MMU
> >   	select MODULES_USE_ELF_RELA if MODULES
> >   	select MODULE_SECTIONS if MODULES
> >   	select OF
> > diff --git a/arch/riscv/include/asm/pgalloc.h b/arch/riscv/include/asm/pgalloc.h
> > index 3c5e3bd15f46..deaf971253a2 100644
> > --- a/arch/riscv/include/asm/pgalloc.h
> > +++ b/arch/riscv/include/asm/pgalloc.h
> > @@ -102,7 +102,10 @@ static inline void __pud_free_tlb(struct mmu_gather *tlb, pud_t *pud,
> >   		struct ptdesc *ptdesc = virt_to_ptdesc(pud);
> >   		pagetable_pud_dtor(ptdesc);
> > -		tlb_remove_page_ptdesc(tlb, ptdesc);
> > +		if (riscv_use_ipi_for_rfence())
> > +			tlb_remove_page_ptdesc(tlb, ptdesc);
> > +		else
> > +			tlb_remove_ptdesc(tlb, ptdesc);
> >   	}
> >   }
> > @@ -136,8 +139,12 @@ static inline void p4d_free(struct mm_struct *mm, p4d_t *p4d)
> >   static inline void __p4d_free_tlb(struct mmu_gather *tlb, p4d_t *p4d,
> >   				  unsigned long addr)
> >   {
> > -	if (pgtable_l5_enabled)
> > -		tlb_remove_page_ptdesc(tlb, virt_to_ptdesc(p4d));
> > +	if (pgtable_l5_enabled) {
> > +		if (riscv_use_ipi_for_rfence())
> > +			tlb_remove_page_ptdesc(tlb, virt_to_ptdesc(p4d));
> > +		else
> > +			tlb_remove_ptdesc(tlb, virt_to_ptdesc(p4d));
> > +	}
> >   }
> >   #endif /* __PAGETABLE_PMD_FOLDED */
> > @@ -169,7 +176,10 @@ static inline void __pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmd,
> >   	struct ptdesc *ptdesc = virt_to_ptdesc(pmd);
> >   	pagetable_pmd_dtor(ptdesc);
> > -	tlb_remove_page_ptdesc(tlb, ptdesc);
> > +	if (riscv_use_ipi_for_rfence())
> > +		tlb_remove_page_ptdesc(tlb, ptdesc);
> > +	else
> > +		tlb_remove_ptdesc(tlb, ptdesc);
> >   }
> >   #endif /* __PAGETABLE_PMD_FOLDED */
> > @@ -180,7 +190,10 @@ static inline void __pte_free_tlb(struct mmu_gather *tlb, pgtable_t pte,
> >   	struct ptdesc *ptdesc = page_ptdesc(pte);
> >   	pagetable_pte_dtor(ptdesc);
> > -	tlb_remove_page_ptdesc(tlb, ptdesc);
> > +	if (riscv_use_ipi_for_rfence())
> > +		tlb_remove_page_ptdesc(tlb, ptdesc);
> > +	else
> > +		tlb_remove_ptdesc(tlb, ptdesc);
> >   }
> >   #endif /* CONFIG_MMU */
> > diff --git a/arch/riscv/include/asm/tlb.h b/arch/riscv/include/asm/tlb.h
> > index 1eb5682b2af6..a0b8b853503f 100644
> > --- a/arch/riscv/include/asm/tlb.h
> > +++ b/arch/riscv/include/asm/tlb.h
> > @@ -10,6 +10,24 @@ struct mmu_gather;
> >   static void tlb_flush(struct mmu_gather *tlb);
> > +#ifdef CONFIG_MMU
> > +#include <linux/swap.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 __tlb_remove_table(void *table)
> > +{
> > +	free_page_and_swap_cache(table);
> > +}
> > +
> > +#endif /* CONFIG_MMU */
> > +
> >   #define tlb_flush tlb_flush
> >   #include <asm-generic/tlb.h>