[PATCH v2] mm: dmapool: use provided gfp flags for all dma_alloc_coherent() calls
gregkh at linuxfoundation.org
Mon Jan 21 16:01:50 EST 2013
On Mon, Jan 21, 2013 at 06:55:25PM +0000, Arnd Bergmann wrote:
> On Monday 21 January 2013, Soeren Moch wrote:
> > On 01/19/13 21:05, Arnd Bergmann wrote:
> > > from the distribution of the numbers, it seems that there is exactly 1 MB
> > > of data allocated between bus addresses 0x1f90000 and 0x1f9ffff, allocated
> > > in individual pages. This matches the size of your pool, so it's definitely
> > > something coming from USB, and no single other allocation, but it does not
> > > directly point to a specific line of code.
> > Very interesting, so this is no fragmentation problem nor something
> > caused by sata or ethernet.
> > > One thing I found was that the ARM dma-mapping code seems buggy in the way
> > > that it does a bitwise and between the gfp mask and GFP_ATOMIC, which does
> > > not work because GFP_ATOMIC is defined by the absence of __GFP_WAIT.
> > >
> > > I believe we need the patch below, but it is not clear to me if that issue
> > > is related to your problem or now.
> > Out of curiosity I checked include/linux/gfp.h. GFP_ATOMIC is defined as
> > __GFP_HIGH (which means 'use emergency pool', and no wait), so this
> > patch should not make any difference for "normal" (GPF_ATOMIC /
> > GFP_KERNEL) allocations, only for gfp_flags accidentally set to zero.
> Yes, or one of the rare cases where someone intentionally does something like
> (GFP_ATOMIC & !__GFP_HIGH) or (GFP_KERNEL || __GFP_HIGH), which are both
> > So, can a new test with this patch help to debug the pool exhaustion?
> Yes, but I would not expect this to change much. It's a bug, but not likely
> the one you are hitting.
> > > So even for a GFP_KERNEL passed into usb_submit_urb, the ehci driver
> > > causes the low-level allocation to be GFP_ATOMIC, because
> > > qh_append_tds() is called under a spinlock. If we have hundreds
> > > of URBs in flight, that will exhaust the pool rather quickly.
> > >
> > Maybe there are hundreds of URBs in flight in my application, I have no
> > idea how to check this.
> I don't know a lot about USB, but I always assumed that this was not
> a normal condition and that there are only a couple of URBs per endpoint
> used at a time. Maybe Greg or someone else with a USB background can
> shed some light on this.
There's no restriction on how many URBs a driver can have outstanding at
once, and if you have a system with a lot of USB devices running at the
same time, there could be lots of URBs in flight depending on the number
of host controllers and devices and drivers being used.
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