linux-kernel - Re: kernFS/sysfs: mmap and vm

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Date:   Tue, 26 Sep 2017 23:31:29 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Federico Vaga <federico.vaga@...a.pv.it>
Cc:     Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: kernFS/sysfs: mmap and vm_operations close

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