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 > <gre...@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? > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
