riscv: mm: Implement pmdp_collapse_flush for THP

When THP is enabled, 4K pages are collapsed into a single huge
page using the generic pmdp_collapse_flush() which will further
use flush_tlb_range() to shoot-down stale TLB entries. Unfortunately,
the generic pmdp_collapse_flush() only invalidates cached leaf PTEs
using address specific SFENCEs which results in repetitive (or
unpredictable) page faults on RISC-V implementations which cache
non-leaf PTEs.

Provide a RISC-V specific pmdp_collapse_flush() which ensures both
cached leaf and non-leaf PTEs are invalidated by using non-address
specific SFENCEs as recommended by the RISC-V privileged specification.

Fixes: e88b333 ("riscv: mm: add THP support on 64-bit")
Signed-off-by: Mayuresh Chitale <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: mdchitale

arch/riscv/include/asm/pgtable.h

@@ -721,6 +721,10 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
 	page_table_check_pmd_set(vma->vm_mm, address, pmdp, pmd);
 	return __pmd(atomic_long_xchg((atomic_long_t *)pmdp, pmd_val(pmd)));
 }
+
+#define pmdp_collapse_flush pmdp_collapse_flush
+extern pmd_t pmdp_collapse_flush(struct vm_area_struct *vma,
+				 unsigned long address, pmd_t *pmdp);
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
 /*

arch/riscv/mm/pgtable.c

@@ -81,3 +81,23 @@ int pmd_free_pte_page(pmd_t *pmd, unsigned long addr)
 }
 
 #endif /* CONFIG_HAVE_ARCH_HUGE_VMAP */
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+pmd_t pmdp_collapse_flush(struct vm_area_struct *vma,
+					unsigned long address, pmd_t *pmdp)
+{
+	pmd_t pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
+
+	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
+	VM_BUG_ON(pmd_trans_huge(*pmdp));
+	/*
+	 * When leaf PTE entries (regular pages) are collapsed into a leaf
+	 * PMD entry (huge page), a valid non-leaf PTE is converted into a
+	 * valid leaf PTE at the level 1 page table.  Since the sfence.vma
+	 * forms that specify an address only apply to leaf PTEs, we need a
+	 * global flush here.  collapse_huge_page() assumes these flushes are
+	 * eager, so just do the fence here.
+	 */
+	flush_tlb_mm(vma->vm_mm);
+	return pmd;
+}
+#endif /* CONFIG_TRANSPARENT_HUGEPAGE */