Hi Qi,
On Tue, Feb 25, 2025 at 11:48 AM Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
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@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxx>
Thanks,
Yunhui
