linux-kernel - [PATCH 16/16] kcov: update kcov to use mmap_prepare, mmap

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Message-ID: <65136c2b1b3647c31bc123a7227263a99112fd44.1757329751.git.lorenzo.stoakes@oracle.com>
Date: Mon,  8 Sep 2025 12:10:47 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Jonathan Corbet <corbet@....net>, Matthew Wilcox <willy@...radead.org>,
        Guo Ren <guoren@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
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        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
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Subject: [PATCH 16/16] kcov: update kcov to use mmap_prepare, mmap_complete

Now we have the capacity to set up the VMA in f_op->mmap_prepare and then
later, once the VMA is established, insert a mixed mapping in
f_op->mmap_complete, do so for kcov.

We utilise the context desc->mmap_context field to pass context between
mmap_prepare and mmap_complete to conveniently provide the size over which
the mapping is performed.

Also note that we intentionally set VM_MIXEDMAP ahead of time so upon
mmap_complete being invoked, vm_insert_page() does not adjust VMA flags.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
---
 kernel/kcov.c | 40 ++++++++++++++++++++++++++++------------
 1 file changed, 28 insertions(+), 12 deletions(-)

diff --git a/kernel/kcov.c b/kernel/kcov.c
index 1d85597057e1..53c8bcae54d0 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -484,23 +484,40 @@ void kcov_task_exit(struct task_struct *t)
 	kcov_put(kcov);
 }
 
-static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
+static int kcov_mmap_prepare(struct vm_area_desc *desc)
 {
-	int res = 0;
-	struct kcov *kcov = vma->vm_file->private_data;
-	unsigned long size, off;
-	struct page *page;
+	struct kcov *kcov = desc->file->private_data;
+	unsigned long size;
 	unsigned long flags;
+	int res = 0;
 
 	spin_lock_irqsave(&kcov->lock, flags);
 	size = kcov->size * sizeof(unsigned long);
-	if (kcov->area == NULL || vma->vm_pgoff != 0 ||
-	    vma->vm_end - vma->vm_start != size) {
+	if (kcov->area == NULL || desc->pgoff != 0 ||
+	    vma_desc_size(desc) != size) {
 		res = -EINVAL;
 		goto exit;
 	}
 	spin_unlock_irqrestore(&kcov->lock, flags);
-	vm_flags_set(vma, VM_DONTEXPAND);
+
+	desc->vm_flags |= VM_DONTEXPAND | VM_MIXEDMAP;
+	desc->mmap_context = (void *)size;
+
+	return 0;
+exit:
+	spin_unlock_irqrestore(&kcov->lock, flags);
+	return res;
+}
+
+static int kcov_mmap_complete(struct file *file, struct vm_area_struct *vma,
+			       const void *context)
+{
+	struct kcov *kcov = file->private_data;
+	unsigned long size = (unsigned long)context;
+	struct page *page;
+	unsigned long off;
+	int res;
+
 	for (off = 0; off < size; off += PAGE_SIZE) {
 		page = vmalloc_to_page(kcov->area + off);
 		res = vm_insert_page(vma, vma->vm_start + off, page);
@@ -509,10 +526,8 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
 			return res;
 		}
 	}
+
 	return 0;
-exit:
-	spin_unlock_irqrestore(&kcov->lock, flags);
-	return res;
 }
 
 static int kcov_open(struct inode *inode, struct file *filep)
@@ -761,7 +776,8 @@ static const struct file_operations kcov_fops = {
 	.open		= kcov_open,
 	.unlocked_ioctl	= kcov_ioctl,
 	.compat_ioctl	= kcov_ioctl,
-	.mmap		= kcov_mmap,
+	.mmap_prepare	= kcov_mmap_prepare,
+	.mmap_complete	= kcov_mmap_complete,
 	.release        = kcov_close,
 };
 
-- 
2.51.0