arm64/mm: implement pte_batch_hint()

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.

Link: https://lkml.kernel.org/r/20240215103205.2607016-17-ryan.roberts@arm.comSigned-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Tested-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Barry Song <21cnbao@gmail.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Morse <james.morse@arm.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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