virt_to_phys_user to print a physical address for a mmap

[balaji-ch]

Hi,
Thanks a lot for the codes that you are sharing. It is helping me to understand Linux better. 🐧

(All these experiments are on Ubuntu VM)
I have used linux-kernel-module-cheat/userland/virt_to_phys_user.c to print a physical address of a variable like this

	int var = 42;
	pid = getpid();//strtoull(argv[1], NULL, 0);
	vaddr = (uintptr_t)&var;//strtoull(argv[2], NULL, 0);

and I get some reasonable values for virtual address, page frame number and physical address as

virtual address , Page frame no, physical address
0x55e451c272a0, 0x3d4a27, 0x3d4a272a0

But when I use it for a mmap I am getting 0 as physical address.

(for this I have compiled the kernel 5.9 with enabling CMA and making CONFIG_STRICT_DEVMEM=n
dmesg | cma
[ 0.068544] cma: Reserved 64 MiB at 0x000000021bc00000

)

fd = open("/dev/mem", O_RDWR | O_SYNC);
vaddr = mmap(0, 64*1024*1024, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0x000000021bc00000);

virtual address , Page frame no, physical address
0x7fd5fffb3000, 0x0, 0x0

Does this program needs some modification for mmap's physical address ? or is there something I am missing ?

[cirosantilli]

Hi Balaji,

So you want to do a mmap and then get its physical address is that right?

But are you trying to the the physical address from:

vaddr = mmap(0, 64*1024*1024, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0x000000021bc00000);

by searching for vaddr to /proc/%ju/pagemap?

If so, I'm not sure that can work, since using /dev/mem for mmap makes mmap enter the magic physical address mode which might not have any corresponding virtual address (presumably implemented by giving the /dev/mem file a custom mmap behaviour with file operations: https://cirosantilli.com/linux-kernel-module-cheat/#file-operations )

[balaji-ch]

So, even though it does not have any corresponding virtual address. Writing to vaddr will reflect in the /dev/mem
for example
vaddr[5000] = 42 will reflect in 0x000000021bc00000+5000 ?

[cirosantilli]

So, even though it does not have any corresponding virtual address. Writing to vaddr will reflect in the /dev/mem
for example
vaddr[5000] = 42 will reflect in 0x000000021bc00000+5000 ?

Yes, I think this is what happens when using the /dev/mem magic mmap.

[balaji-ch]

Thank you for your swift replies.
AlsoI have one more question, In many examples thatI have seen, the size was always 1 page table. Can we use. 64 MiB for sizes?

[cirosantilli]

You mean the mmap size as in mmap(0, 64*1024*1024? I think any size should be fine. man mmap does not seem to constrain it to even be a multiple of pages.man mmap also mentions MAP_HUGETLB which might be related to what you have in mind.

[balaji-ch]

In many examples that mmap a physical memory I have seen the size was page_size in general.

for instance here
https://github.com/Digilent/Petalinux-Arty-Z7-20/blob/master/Arty-Z7-20/project-spec/meta-user/recipes-apps/peekpoke/files/peek.c

ptr=mmap(NULL,page_size,PROT_READ,MAP_SHARED,fd,(addr & ~(page_size-1)));

So, I was wondering if there is a limitation for mmaping physical memory.