Message-Id: <86e9fbf586912f3689e98c4dee2180b69b399638.1737637631.git.zhengqi.arch@bytedance.com>
Date: Thu, 23 Jan 2025 21:26:16 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
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	Qi Zheng <zhengqi.arch@...edance.com>
Subject: [PATCH 3/5] riscv: pgtable: unconditionally use tlb_remove_ptdesc()

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@...edance.com>
Suggested-by: Peter Zijlstra (Intel) <peterz@...radead.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

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