Re: USBFS Memory allocation Bug

[Alan Stern]

On Wed, 10 Mar 2010, Mel Gorman wrote:

> On Wed, Mar 10, 2010 at 10:16:09AM -0500, Alan Stern wrote:
> > On Wed, 10 Mar 2010, Mel Gorman wrote:
> > 
> > > On Tue, Mar 09, 2010 at 10:09:57PM +0100, Markus Rechberger wrote:
> > > > Hi,
> > > > 
> > > > I just stumbled around following issue:
> > > > 
> > > > <SNIP to have one full message>
> > > >
> > > > [275835.491094] mediasrv: page allocation failure. order:6, mode:0x40d0
> > > 
> > > This is massive. Why is usbdev_ioctl requiring an order-6 allocation?
> > > Does it have the option to fall back to vmalloc() for example?
> > 
> > We needed to implement an upper limit on the buffer size, and the value
> > chosen was sufficient for 8 ms of data.  There is no fallback to
> > vmalloc().
> > 
> > Of course, the mediasrv program could always make multiple smaller
> > data submissions instead of a single big one.
> > 
> 
> Is there any means for the driver to take the large request, break it up
> into multiple smaller requests and submit them one at a time?

In theory almost anything is possible.  But it would be a big effort
and not consistent with the way the rest of the driver works.

> > > So, it's a high-order allocation but no swap is configured. As the
> > > system has a big mix of both anonymous and file memory, it is likely it
> > > was unable to find a large enough contiguous range of file-backed memory
> > > it could discard. There is pretty much nothing the memory manager could
> > > do here. 
> > > 
> > > FWIW, if swap was configured I'd think there was more the memory manager
> > > could have done.
> > > 
> > > Is usb falling back to vmalloc() or order-0 pages possible?
> > 
> > No.  It's not possible since the buffer has to be contiguous for DMA
> > purposes (this is a requirement of the driver interface).
> > 
> 
> Regrettably, there is not much the VM can do in this situation. Without
> swap, it cannot magic up contiguous memory. If memory compaction gets
> merged then it could do more but the risk of failure is still non-zero.

Agreed.  The best way to fix the allocation failure is to provide swap 
space.

Alan Stern

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