Re: [PATCH] swiotlb: Fix inversed DMA_ATTR_NO_WARN test

[Christian KÃnig]

Am 02.05.2018 um 18:59 schrieb Michel DÃnzer:
> On 2018-05-02 06:21 PM, Christoph Hellwig wrote:
> > On Wed, May 02, 2018 at 04:31:09PM +0200, Michel DÃnzer wrote:
> > > > No.  __GFP_NOWARN (and gfp_t flags in general) are the wrong interface
> > > > for dma allocations and just cause problems.  I actually plan to
> > > > get rid of the gfp_t argument in dma_alloc_attrs sooner, and only
> > > > allow either GFP_KERNEL or GFP_DMA passed in dma_alloc_coherent.
> > > How about GFP_TRANSHUGE_LIGHT? TTM uses that to opportunistically
> > > allocate huge pages (GFP_TRANSHUGE can result in unacceptably long
> > > delays with memory pressure).
> > Well, that is exactly what I don't want drivers to do - same for
> > __GFP_COMP in some drm code.  This very much assumes the page allocator
> > is used to back dma allocations, which very often it actually isn't, and
> > any use of magic gfp flags creates a tight coupling of consumers with a
> > specific implementation.
> >
> > In general I can't think of a good reason not to actually use
> > GFP_TRANSHUGE_LIGHT by default in the dma allocator unless
> > DMA_ATTR_ALLOC_SINGLE_PAGES is set.  Can you prepare a patch for that?
> I'm afraid I'll have to leave that to somebody else.

Coming back to this topic once more, sorry for the delay but busy as 
usual :)

What exactly do you mean with "dma allocator" here? The TTM allocator 
using the dma_alloc_coherent calls? Or the swiotlb implementation of the 
calls?

Christian.