Re: bade page state while calling munmap() for kmalloc'ed UIO memory

["Hans J. Koch" <hjk@hansjkoch.de>]

On Wed, Aug 31, 2011 at 04:13:07PM -0700, Andrew Morton wrote:
> On Wed, 31 Aug 2011 11:58:25 +0200
> "Hans J. Koch" <hjk@hansjkoch.de> wrote:
> 
> > On Mon, Aug 29, 2011 at 05:05:47PM +0200, Jan Altenberg wrote:
> > 
> > [Since we got no reply on linux-mm, I added lkml and Andrew to Cc: (mm doesn't
> > seem to have a maintainer...)]
> > 
> > > Hi,
> > > 
> > > I'm currently analysing a problem similar to some mmap() issue reported
> > > in the past: https://lkml.org/lkml/2010/7/11/140
> > 
> > The arch there was microblaze, and you are working on arm. That means
> > the problem appears on at least to archs.
> > 
> > > 
> > > So, what I'm trying to do is mapping some physically continuous memory
> > > (allocated by kmalloc) to userspace, using a trivial UIO driver (the
> > > idea is that a device can directly DMA to that buffer):
> > > 
> > > [...]
> > > #define MEM_SIZE (4 * PAGE_SIZE)
> > > 
> > > addr = kmalloc(MEM_SIZE, GFP_KERNEL)
> > > [...]
> > > info.mem[0].addr = (unsigned long) addr;
> > > info.mem[0].internal_addr = addr;
> > > info.mem[0].size = MEM_SIZE;
> > > info.mem[0].memtype = UIO_MEM_LOGICAL;
> > > [...]
> > > ret = uio_register_device(&pdev->dev, &info);
> > > 
> > > Userspace maps that memory range and writes its contents to a file:
> > > 
> > > [...]
> > > 
> > > fd = open("/dev/uio0", O_RDWR);
> > > if (fd < 0) {
> > >            perror("Can't open UIO device\n");
> > >            exit(1);
> > > }
> > > 
> > > mem_map = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE,
> > >                   MAP_PRIVATE, fd, 0);
> > > 
> > > if(mem_map == MAP_FAILED) {
> > >            perror("Can't map UIO memory\n");
> > >            ret = -ENOMEM;
> > >            goto out_file;
> > > }
> > > [...]
> > > bytes_written = write(fd_file, mem_map, MAP_SIZE)
> > > [...]
> > > 
> > > munmap(mem_map);
> > 
> > >From my point of view (I've got Jan's full test case code), this
> > is a completely correct UIO use case.
> > 
> > > 
> > > So, what happens is (I'm currently testing with 3.0.3 on ARM
> > > VersatilePB): When I do the munmap(), I run into the following error:
> > > 
> > > BUG: Bad page state in process uio_test  pfn:078ed
> > > page:c0409154 count:0 mapcount:0 mapping:  (null) index:0x0
> > > page flags: 0x284(referenced|slab|arch_1)
> 
> PG_slab is set.  The kernel is complaining because a page which was
> allocated via kmalloc/kmem_cache_alloc was directly passed to the page
> allocator for freeing.  It should have been passed to kfree().
> 
> Presumably the uio driver expects that its memory was allocated via
> alloc_pages(), not via kmalloc().

Thanks for that hint. I'll check and will update UIO documentation
accordingly if necessary.

BUT the following problems are still threatening my mental health:

If userspace gets a valid and working pointer from mmap(), it is
entitled to expect munmap() to work on that pointer, too, isn't it?
Shouldn't mmap() fail in such a case?

The kernel's behavior should be the same, no matter which SLAB or
SLUB is chosen.

Or am I following wrong philosophies?

Thanks,
Hans


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