| Message ID | 9025595e895515515c95e48db54b29afa489c41d.1740454179.git.zhengqi.arch@bytedance.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | remove tlb_remove_page_ptdesc() | expand |
Hi Qi, On Tue, Feb 25, 2025 at 11:48 AM Qi Zheng <zhengqi.arch@bytedance.com> 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 <zhengqi.arch@bytedance.com> > Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> > --- > 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 <cuiyunhui@bytedance.com> 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 <cuiyunhui@bytedance.com> Thanks, Yunhui
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 */
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 <zhengqi.arch@bytedance.com> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- arch/riscv/include/asm/pgalloc.h | 26 ++++---------------------- 1 file changed, 4 insertions(+), 22 deletions(-)