Mapping IO memory read-write

Jethro Beekman kernel at
Tue Jan 12 18:10:23 EST 2016

I'm writing a device driver for a memory-mapped device on x86-64. I'm mapping
the device in the kernel using ioremap_cache(). My file_operations.mmap function
is as follows:

static int dev_mmap(struct file *filep, struct vm_area_struct *vma) {
	return vm_iomap_memory(vma,START,LEN);

The user process calls mmap(..,PROT_READ|PROT_WRITE|PROT_EXEC,..). The mapping
in /proc/[pid]/maps shows the write bit. However, when looking at the actual
page table entry does not have the RW bit set. For example:

virtual address = 0000_0001_8000_0000
cr3 = 0000_0000_700a_e000
phys:0000_0000_700a_e000 = 0000_0000_7e05_5067 (P|RW|US|A|D)
phys:0000_0000_7e05_5030 = 0000_0001_66e2_9067 (P|RW|US|A|D)
phys:0000_0001_66e2_9000 = 0000_0001_69db_3067 (P|RW|US|A|D)
phys:0000_0001_69db_3000 = 0000_0000_8020_0225 (P|US|A|SOFTW1)

Am I doing something wrong?


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