Re: kernFS/sysfs: mmap and vm_operations close

Greg Kroah-Hartman
2017-09-26T21:31:22Z

On Tue, Sep 26, 2017 at 05:50:55PM +0200, Federico Vaga wrote:
> Hello,
>
> I'm writing a sysfs binary attribute that makes use of the `mmap` operation.

Eeek, why? What are you using that for?

sysfs binary attributes are for dumping binary data that the kernel
doesn't touch/parse, through to hardware. Why use mmap for this? Do
you have a pointer to your code somewhere?

> I would like to implement my own `open()` and `close()` `vm_ops` but
> apparently I'm not allowed to do it.

Nope, you are not, not in sysfs :)

>
> -------- 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" ?

Because you shouldn't be doing that :)

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

Don't use mmap in sysfs :)

> 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`

Don't use mmap in sysfs :)

What problem are you trying to solve here that mmap seemed like the
correct solution?

thanks,

greg k-h