/dev/zero: try to align PMD_SIZE for private mapping

Attempt to map aligned to huge page size for private mapping which could
achieve performance gains, the mprot_tw4m in libMicro average execution
time on arm64:

- Test case: mprot_tw4m
- Before the patch: 22 us
- After the patch: 17 us

If THP config is not set, we fall back to system page size mappings.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Zhang Qilong <[e-mail]>
Reviewed-by: Lorenzo Stoakes <[e-mail]>
Tested-by: Lorenzo Stoakes <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Kefeng Wang <[e-mail]>
Cc: Nanyong Sun <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

drivers/char/mem.c

···
 	return 0;
 }
 
+#ifndef CONFIG_MMU
 static unsigned long get_unmapped_area_zero(struct file *file,
 				unsigned long addr, unsigned long len,
 				unsigned long pgoff, unsigned long flags)
 {
-#ifdef CONFIG_MMU
+	return -ENOSYS;
+}
+#else
+static unsigned long get_unmapped_area_zero(struct file *file,
+				unsigned long addr, unsigned long len,
+				unsigned long pgoff, unsigned long flags)
+{
 	if (flags & MAP_SHARED) {
 		/*
 		 * mmap_zero() will call shmem_zero_setup() to create a file,
···
 		return shmem_get_unmapped_area(NULL, addr, len, pgoff, flags);
 	}
 
-	/* Otherwise flags & MAP_PRIVATE: with no shmem object beneath it */
-	return mm_get_unmapped_area(current->mm, file, addr, len, pgoff, flags);
+	/*
+	 * Otherwise flags & MAP_PRIVATE: with no shmem object beneath it,
+	 * attempt to map aligned to huge page size if possible, otherwise we
+	 * fall back to system page size mappings.
+	 */
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	return thp_get_unmapped_area(file, addr, len, pgoff, flags);
 #else
-	return -ENOSYS;
+	return mm_get_unmapped_area(current->mm, file, addr, len, pgoff, flags);
 #endif
 }
+#endif /* CONFIG_MMU */
 
 static ssize_t write_full(struct file *file, const char __user *buf,
 			  size_t count, loff_t *ppos)