[PATCH v3] /dev/zero: try to align PMD_SIZE for private mapping

[Zhang Qilong <zhangqilong3@huawei.com>]

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.

Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
---
v3:
- collect Acked-by
- factor out the #ifdef CONFIG_MMU in get_unmapped_area_zero(), per Lorenzo
- explicitly use #ifdef CONFIG_TRANSPARENT_HUGEPAGE, per Lorenzo and Matthew

v2:
- add comments on code suggested by Lorenzo
- use IS_ENABLED to check THP config

 drivers/char/mem.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 48839958b0b1..b7c4dbe4d2c1 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -510,31 +510,44 @@ static int mmap_zero(struct file *file, struct vm_area_struct *vma)
 		return shmem_zero_setup(vma);
 	vma_set_anonymous(vma);
 	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)
+{
+	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)
 {
-#ifdef CONFIG_MMU
 	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(),
 		 * 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 */
-	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
 
 static ssize_t write_full(struct file *file, const char __user *buf,
 			  size_t count, loff_t *ppos)
 {
 	return -ENOSPC;
-- 
2.43.0