linux-kernel - [PATCH] /dev/zero: make private mapping full anonymous mapping

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Message-ID: <20250113223033.4054534-1-yang@os.amperecomputing.com>
Date: Mon, 13 Jan 2025 14:30:33 -0800
From: Yang Shi <yang@...amperecomputing.com>
To: arnd@...db.de,
	gregkh@...uxfoundation.org,
	Liam.Howlett@...cle.com,
	lorenzo.stoakes@...cle.com,
	vbabka@...e.cz,
	jannh@...gle.com,
	willy@...radead.org,
	liushixin2@...wei.com,
	akpm@...ux-foundation.org
Cc: yang@...amperecomputing.com,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] /dev/zero: make private mapping full anonymous mapping

When creating private mapping for /dev/zero, the driver makes it an
anonymous mapping by calling set_vma_anonymous().  But it just sets
vm_ops to NULL, vm_file is still valid and vm_pgoff is also file offset.

This is a special case and the VMA doesn't look like either anonymous VMA
or file VMA.  It confused other kernel subsystem, for example, khugepaged [1].

It seems pointless to keep such special case.  Making private /dev/zero
mapping a full anonymous mapping doesn't change the semantic of
/dev/zero either.

The user visible effect is the mapping entry shown in /proc/<PID>/smaps
and /proc/<PID>/maps.

Before the change:
ffffb7190000-ffffb7590000 rw-p 00001000 00:06 8                          /dev/zero

After the change:
ffffb6130000-ffffb6530000 rw-p 00000000 00:00 0

[1]: https://lore.kernel.org/linux-mm/20250111034511.2223353-1-liushixin2@huawei.com/

Signed-off-by: Yang Shi <yang@...amperecomputing.com>
---
 drivers/char/mem.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 169eed162a7f..dae113f7fc1b 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -527,6 +527,10 @@ static int mmap_zero(struct file *file, struct vm_area_struct *vma)
 	if (vma->vm_flags & VM_SHARED)
 		return shmem_zero_setup(vma);
 	vma_set_anonymous(vma);
+	fput(vma->vm_file);
+	vma->vm_file = NULL;
+	vma->vm_pgoff = vma->vm_start >> PAGE_SHIFT;
+
 	return 0;
 }

-- 
2.47.0