Re: Unmapping of UIO logical memory causing a trace

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Aug 19, 2015 at 03:25:09PM +0530, Ankit Jindal wrote:
> Hi Greg,
> 
> On Wed, Aug 12, 2015 at 10:41 PM, Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> > On Wed, Aug 12, 2015 at 01:25:53PM +0530, Ankit Jindal wrote:
> >> Hi Greg,
> >>
> >> On Wed, Aug 12, 2015 at 3:15 AM, Greg Kroah-Hartman
> >> <gregkh@linuxfoundation.org> wrote:
> >> > On Tue, Aug 11, 2015 at 04:39:08PM +0530, Ankit Jindal wrote:
> >> >> Hi,
> >> >>
> >> >> We have observed an issue where kmalloc of a small sized memory causes
> >> >> an occasional trace when unmapping the mmaped memory via UIO framework
> >> >> This trace is coming when kernel sees a negative value in
> >> >> page->_mapcount. Trace is pasted at the end of the mail.
> >> >>
> >> >> After debugging this issue further, we realized following sequence
> >> >> occurs when kmalloc is used to allocate small memory using slub
> >> >> allocator:
> >> >> 1. Frozen bit (msb) of the page from which memory has been allocated
> >> >> is set (which is an union with _mapcount).
> >> >> 2. If there are free objects in the the same page then this frozen bit
> >> >> remains set even after kernel boots completely.
> >> >> 3. When user space calls unmap of this memory, vma_unmap_single()
> >> >> treats the _mapcount as a negative (as frozen bit is set), causing a
> >> >> trace.
> >> >>
> >> >> We are not sure whether exposing kernel memory of size
> >> >> less than PAGE_SIZE via UIO is a valid use case ? In case this is an invalid
> >> >> use case then shouldn't the UIO framework restrict mapping of non
> >> >> PAGE_SIZE aligned memory and size not in order of PAGE_SIZE.
> >> >
> >> > We've had a few discussions about this in the past, and one proposed
> >> > patch which had to be reverted because it broke some working systems, so
> >> > it's a messy thing.
> >> >
> >> > What UIO driver are you using that causes this behavior?
> >>
> >> We have observed this during the development of new UIO driver for our
> >> soc. In our driver, we need to parse non probable properties of device
> >> and provide these details to our user application.
> >
> > What exactly do you mean by this?
> 
> There are certain the device specific properties (coming from device
> tree) which we are exposing using UIO_LOGICAL_MEM to the user space.

Why would you do that?  Userspace has the device tree info already
through other means.

> >> For this we do a kmalloc of device info(approx size 80 bytes) and pass
> >> this address to user space via UIO mem logical.
> >
> > For such tiny sizes, why not just use a normal sysfs file?
> 
> We did not think of sysfs as an option, since UIO_MEM_LOGICAL seems to
> address our use case.

Not for tiny memory chunks like you have seen :)

> > Do you have a pointer to your driver so that I can see exactly what it
> > is doing here?
> 
> Below is the code snippet of my driver.

<snip>

How about the whole driver please.

thanks,

greg k-h