Re: Unmapping of UIO logical memory causing a trace

[Greg Kroah-Hartman]

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@xxxxxxxxxxxxxxxxxxx> 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?

> 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?

Do you have a pointer to your driver so that I can see exactly what it
is doing here?

thanks,

greg k-h
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