Re: [LSF/MM TOPIC NOTES] x86 ZONE_DMA love

[Matthew Wilcox <willy6545@gmail.com>]

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Another way we could approach this is to get rid of ZONE_DMA. Make GFP_DMA
a flag which doesn't map to a zone. Rather, it redirects to a separate
allocator. At boot, we hand all memory under 16MB to the DMA allocator. The
DMA allocator can have a shrinker which just hands back all the memory once
we're under memory pressure (if it's never had an allocation).

I think we can get rid of the GFP_DMA support in slab/slub. We still need
to be able to allocate pages to support bounce buffers / dma_alloc_foo, but
there's really no reason to allocate sub-pages at this point.

On Sat, Apr 28, 2018, 14:55 Luis R. Rodriguez, <mcgrof@kernel.org> wrote:

> On Sat, Apr 28, 2018 at 01:42:21AM -0700, Christoph Hellwig wrote:
> > On Fri, Apr 27, 2018 at 04:14:56PM +0000, Luis R. Rodriguez wrote:
> > > Do we have a list of users for x86 with a small DMA mask?
> > > Or, given that I'm not aware of a tool to be able to look
> > > for this in an easy way, would it be good to find out which
> > > x86 drivers do have a small mask?
> >
> > Basically you'll have to grep for calls to dma_set_mask/
> > dma_set_coherent_mask/dma_set_mask_and_coherent and their pci_*
> > wrappers with masks smaller 32-bit.  Some use numeric values,
> > some use DMA_BIT_MASK and various places uses local variables
> > or struct members to parse them, so finding them will be a bit
> > more work.  Nothing a coccinelle expert couldn't solve, though :)
>
> Thing is unless we have a specific flag used consistently I don't believe
> we
> can do this search with Coccinelle. ie, if we have local variables and
> based on
> some series of variables things are set, this makes the grammatical
> expression
> difficult to express.  So Cocinelle is not designed for this purpose.
>
> But I believe smatch [0] is intended exactly for this sort of purpose, is
> that
> right Dan? I gave a cursory look and I think it'd take me significant time
> to
> get such hunt down.
>
> [0] https://lwn.net/Articles/691882/
>
>   Luis
>

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