Re: [PATCH 3/5] dma-direct: refine dma_direct_alloc zone selection

[Christoph Hellwig <hch@lst.de>]

On Fri, Sep 28, 2018 at 10:06:48AM +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2018-09-27 at 15:49 +0200, Christoph Hellwig wrote:
> > On Thu, Sep 27, 2018 at 11:45:15AM +1000, Benjamin Herrenschmidt wrote:
> > > I'm not sure this is entirely right.
> > > 
> > > Let's say the mask is 30 bits. You will return GFP_DMA32, which will
> > > fail if you allocate something above 1G (which is legit for
> > > ZONE_DMA32).
> > 
> > And then we will try GFP_DMA further down in the function:
> > 
> > 		if (IS_ENABLED(CONFIG_ZONE_DMA) &&
> > 		    dev->coherent_dma_mask < DMA_BIT_MASK(32) &&
> > 		    !(gfp & GFP_DMA)) {
> > 			gfp = (gfp & ~GFP_DMA32) | GFP_DMA;
> > 			goto again;
> > 		}
> > 
> > This is and old optimization from x86, because chances are high that
> > GFP_DMA32 will give you suitable memory for the infamous 31-bit
> > dma mask devices (at least at boot time) and thus we don't have
> > to deplete the tiny ZONE_DMA pool.
> 
> I see, it's rather confusing :-) Wouldn't it be better to check against
> top of 32-bit memory instead here too ?

Where is here?  In __dma_direct_optimal_gfp_mask we already handled
it due to the optimistic zone selection we are discussing.

In the fallback quoted above there is no point for it, as with a
physical memory size smaller than ZONE_DMA32 (or ZONE_DMA for that matter)
we will have succeeded with the optimistic zone selection and not hit
the fallback path.

Either way this code probably needs much better comments.  I'll send
a patch on top of the recent series.