[PATCH resend v2] /dev/zero: try to align PMD_SIZE for private mapping

[Zhang Qilong]

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

Signed-off-by: Zhang Qilong <zhangqilong3@xxxxxxxxxx>
---
v2:
- Add comments on code suggested by Lorenzo
- Use IS_ENABLED to check THP config

 drivers/char/mem.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 48839958b0b1..c27cc89bd02d 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -525,11 +525,18 @@ static unsigned long get_unmapped_area_zero(struct file *file,
 		 * so as not to confuse shmem with our handle on "/dev/zero".
 		 */
 		return shmem_get_unmapped_area(NULL, addr, len, pgoff, flags);
 	}
 
-	/* Otherwise flags & MAP_PRIVATE: with no shmem object beneath it */
+	/*
+	 * 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.
+	 */
+	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
+		return thp_get_unmapped_area(file, addr, len, pgoff, flags);
+
 	return mm_get_unmapped_area(current->mm, file, addr, len, pgoff, flags);
 #else
 	return -ENOSYS;
 #endif
 }
-- 
2.43.0