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

[Andi Kleen]

Yasunori Goto <y-goto@xxxxxxxxxxxxxx> writes:
>
> 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.

The VM has special "lower zone protection" to protect against these
kinds of deadlocks. They can be circumvented, but it takes effort.

>   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.

Normally mlock is limited to half the memory exactly to avoid
such problems.

Also I believe there are some issues with non continuous memory on
some IA64 systems -- e.g. Altixes iirc have the requirement
that you use the IOMMU for higher memory. So it's probably
not easy to change.

-Andi

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