[PATCH v6 16/18] arm64/mm: Implement pte_batch_hint()

[Ryan Roberts]

When core code iterates over a range of ptes and calls ptep_get() for
each of them, if the range happens to cover contpte mappings, the number
of pte reads becomes amplified by a factor of the number of PTEs in a
contpte block. This is because for each call to ptep_get(), the
implementation must read all of the ptes in the contpte block to which
it belongs to gather the access and dirty bits.

This causes a hotspot for fork(), as well as operations that unmap
memory such as munmap(), exit and madvise(MADV_DONTNEED). Fortunately we
can fix this by implementing pte_batch_hint() which allows their
iterators to skip getting the contpte tail ptes when gathering the batch
of ptes to operate on. This results in the number of PTE reads returning
to 1 per pte.

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Tested-by: John Hubbard <jhubbard@xxxxxxxxxx>
Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
---
 arch/arm64/include/asm/pgtable.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index a8f1a35e3086..d759a20d2929 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -1213,6 +1213,15 @@ static inline void contpte_try_unfold(struct mm_struct *mm, unsigned long addr,
 		__contpte_try_unfold(mm, addr, ptep, pte);
 }
 
+#define pte_batch_hint pte_batch_hint
+static inline unsigned int pte_batch_hint(pte_t *ptep, pte_t pte)
+{
+	if (!pte_valid_cont(pte))
+		return 1;
+
+	return CONT_PTES - (((unsigned long)ptep >> 3) & (CONT_PTES - 1));
+}
+
 /*
  * The below functions constitute the public API that arm64 presents to the
  * core-mm to manipulate PTE entries within their page tables (or at least this
-- 
2.25.1