On Thu 01-12-16 17:03:52, Michal Nazarewicz wrote:
> On Thu, Dec 01 2016, Michal Hocko wrote:
> > Let's also CC Marek
> >
> > On Thu 01-12-16 08:43:40, Vlastimil Babka wrote:
> >> On 12/01/2016 08:21 AM, Michal Hocko wrote:
> >> > Forgot to CC Joonsoo. The email thread starts more or less here
> >> > http://lkml.kernel.org/r/20161130092239.gd18...@dhcp22.suse.cz
> >> > 
> >> > On Thu 01-12-16 08:15:07, Michal Hocko wrote:
> >> > > On Wed 30-11-16 20:19:03, Robin H. Johnson wrote:
> >> > > [...]
> >> > > > alloc_contig_range: [83f2a3, 83f2a4) PFNs busy
> >> > > 
> >> > > Huh, do I get it right that the request was for a _single_ page? Why do
> >> > > we need CMA for that?
> >> 
> >> Ugh, good point. I assumed that was just the PFNs that it failed to migrate
> >> away, but it seems that's indeed the whole requested range. Yeah sounds 
> >> some
> >> part of the dma-cma chain could be smarter and attempt CMA only for e.g.
> >> costly orders.
> >
> > Is there any reason why the DMA api doesn't try the page allocator first
> > before falling back to the CMA? I simply have a hard time to see why the
> > CMA should be used (and fragment) for small requests size.
> 
> There actually may be reasons to always go with CMA even if small
> regions are requested.  CMA areas may be defined to map to particular
> physical addresses and given device may require allocations from those
> addresses.  This may be more than just a matter of DMA address space.
> I cannot give you specific examples though and I might be talking
> nonsense.

I am not familiar with this code so I cannot really argue but a quick
look at rmem_cma_setup doesn't suggest any speicific placing or
anything...

-- 
Michal Hocko
SUSE Labs

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