riscv: pgtable: unconditionally use tlb_remove_ptdesc()

To support fast gup, the commit 69be3fb ("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.

Link: https://lkml.kernel.org/r/9025595e895515515c95e48db54b29afa489c41d.1740454179.git.zhengqi.arch@bytedance.com
Signed-off-by: Qi Zheng <[email protected]>
Suggested-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Alexandre Ghiti <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Hugh Dickens <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Kevin Brodsky <[email protected]>
Cc: Matthew Wilcow (Oracle) <[email protected]>
Cc: "Mike Rapoport (IBM)" <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Vishal Moola (Oracle) <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Yu Zhao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Qi Zheng

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 */
 
@@ -143,15 +125,15 @@ 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 */
 
 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 */