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Subject[Q] Why does dma_alloc_coherent() of ia64 GFP_DMA?


I have a (may be dumb) question about dma_alloc_coherent() for ia64.

Why does dma_alloc_coherent() of ia64 force GFP_DMA yet?
And why is swiotlb_dma_alloc_coherent() default routine of

#define dma_alloc_coherent(dev, size, handle, gfp) \
platform_dma_alloc_coherent(dev, size, handle, (gfp) | GFP_DMA)

Even if a device allows over 4G access and the driver doesn't specify
GFP_DMA, dma_alloc_coherent() returns under 4G area.
I guess many people think this is not so big issue because drivers require
very small memory generally.

However, I think this has the possibility of a finishing blow of OOM.
For example,

1) Page caches occupy normal zone, and DMA zone is free.
2) A user's application requires a few GB memory and mlock it.
All DMA zone is occupied by it.
3) A device which allows over 4GB is hot-added.
But dma_alloc_coherent() try to allocate DMA zone.
Then OOM occurs because there is no freeable pages.

I heard there are some users who require a few GB mlock.
There are similar trouble in past.

If GFP_DMA is removed from above definition of dma_alloc_coherent(),
what will happen?

If it is not allowed, how is followings?

-> platform_dma_alloc_coherent()
-> normal_alloc_coherent()
if (dma_mask allow over 4G)
ret = __get_free_pages();
(check validation of returned address)

If I'm something misunderstanding, please let me know.


Yasunori Goto

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