Re: [discuss] Re: 32-bit dma allocations on 64-bit platforms

[Takashi Iwai <tiwai@suse.de>]

At Thu, 24 Jun 2004 19:16:20 +0200,
Andrea Arcangeli wrote:
> 
> On Thu, Jun 24, 2004 at 06:04:58PM +0200, Takashi Iwai wrote:
> > At Thu, 24 Jun 2004 17:29:46 +0200,
> > Andrea Arcangeli wrote:
> > > 
> > > On Thu, Jun 24, 2004 at 04:58:24PM +0200, Takashi Iwai wrote:
> > > > At Thu, 24 Jun 2004 16:42:58 +0200,
> > > > Andi Kleen wrote:
> > > > > 
> > > > > On Thu, 24 Jun 2004 16:36:47 +0200
> > > > > Takashi Iwai <tiwai@suse.de> wrote:
> > > > > 
> > > > > > At Thu, 24 Jun 2004 13:29:00 +0200,
> > > > > > Andi Kleen wrote:
> > > > > > > 
> > > > > > > > Can't it be called with GFP_KERNEL at first, then with GFP_DMA if the
> > > > > > > > allocated pages are out of dma mask, just like in pci-gart.c?
> > > > > > > > (with ifdef x86-64)
> > > > > > > 
> > > > > > > That won't work reliable enough in extreme cases.
> > > > > > 
> > > > > > Well, it's not perfect, but it'd be far better than GFP_DMA only :)
> > > > > 
> > > > > The only description for this patch I can think of is "russian roulette" 
> > > > 
> > > > Even if we have a bigger DMA zone, it's no guarantee that the obtained
> > > > page is precisely in the given mask.  We can unlikely define zones
> > > > fine enough for all different 24, 28, 29, 30 and 31bit DMA masks.
> > > > 
> > > > 
> > > > My patch for i386 works well in most cases, because such a device is
> > > > usually equipped on older machines with less memory than DMA mask.
> > > > 
> > > > Without the patch, the allocation is always <16MB, may fail even small
> > > > number of pages.
> > > 
> > > why does it fail? note that with the lower_zone_reserve_ratio algorithm I
> > > added to 2.4 all dma zone will be reserved for __GFP_DMA allocations so
> > > you should have troubles only with 2.6, 2.4 should work fine.
> > > So with latest 2.4 it has to fail only if you already allocated 16M with
> > > pci_alloc_consistent which sounds unlikely.
> > 
> > If a driver needs large contiguous (e.g. a coule of MB) pages and the
> > memory is fragmented, it may still fail.  But it's anyway very
> > rare...
> 
> Yes. This is why I suggested to use GFP_KERNEL _after_ GFP_DMA has
> failed, not the other way around. As Andi said in big systems you're
> pretty much guaranteed that GFP_KERNEL will always fail.

Ok.

> > However, 16MB isn't enough in some cases indeed.  For example, the
> > following devices are often problematic:
> > 
> > - SB Live (emu10k1)
> >   This needs many single pages for WaveTable synthesis per user's
> >   request (up to 128MB).  It sets 31bit DMA mask (sigh...)
> 
> then it may never work. If the lowmem below 4G is all allocated in
> anonymous memory and you've no swap, there's no way, absolutely no way
> to make the above work. I start to think you should fail insmod if the
> machine has more than 2^31 bytes of ram being used by the kernel.
>
> All we can do is to give it a chance to work, that is to call GFP_KERNEL
> _after_ GFP_DMA has failed, but again  there's no guarantee that it will
> work, even if you've only a few gigs of ram.

Sure, in extreme cases, it can't work.  But at least, it _may_ work
better than using only GFP_DMA.  And indeed it should (still) work
on most of consumer PC boxes.  The addition of another zone would help
much better, though.


Takashi