[PATCH] /dev/zero: try to align PMD_SIZE for private mapping

[Zhang Qilong <zhangqilong3@huawei.com>]

By default, THP are usually enabled. Mapping /dev/zero with a size
larger than 2MB could achieve performance gains by allocating aligned
address. The mprot_tw4m in libMicro average execution time on arm64:
  - Test case:        mprot_tw4m
  - Before the patch:   22 us
  - After the patch:    17 us

Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
---
 drivers/char/mem.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 48839958b0b1..c57327ca9dd6 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -515,10 +515,12 @@ static int mmap_zero(struct file *file, struct vm_area_struct *vma)
 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
+	unsigned long ret;
+
 	if (flags & MAP_SHARED) {
 		/*
 		 * mmap_zero() will call shmem_zero_setup() to create a file,
 		 * so use shmem's get_unmapped_area in case it can be huge;
 		 * and pass NULL for file as in mmap.c's get_unmapped_area(),
@@ -526,10 +528,13 @@ static unsigned long get_unmapped_area_zero(struct file *file,
 		 */
 		return shmem_get_unmapped_area(NULL, addr, len, pgoff, flags);
 	}
 
 	/* Otherwise flags & MAP_PRIVATE: with no shmem object beneath it */
+	ret = thp_get_unmapped_area(file, addr, len, pgoff, flags);
+	if (ret)
+		return ret;
 	return mm_get_unmapped_area(current->mm, file, addr, len, pgoff, flags);
 #else
 	return -ENOSYS;
 #endif
 }
-- 
2.43.0