kernFS/sysfs: mmap and vm_operations close

Federico Vaga
2017-09-26T15:59:26Z

Hello,

I'm writing a sysfs binary attribute that makes use of the `mmap` operation.
I would like to implement my own `open()` and `close()` `vm_ops` but
apparently I'm not allowed to do it.

-------- kernfs/file.c - kernfs_fop_mmap () - modern kernel -----
-------- sysfs/bin.c - mmap () - old kernel -----

/*
* It is not possible to successfully wrap close.
* So error if someone is trying to use close.
*/
rc = -EINVAL;
if (vma->vm_ops && vma->vm_ops->close)
goto out_put;
----------------------------------------------------------

What is the reason behind this choice?
Why "it is not possible to successfully wrap close" ?

Is there an alternative/hack in order to be notified when the mmap is not used
anymore and I can properly release my resources?

Due to HW resources limitation I "cannot" keep the device memory mapped when
nobody is using it, that's why I would like to be able to use vm_ops->close().
In general, I would like to run my routine that release resources when the
user does `munmap` or `close`

thank you very much :)