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SubjectRe: [bug] __blk_mq_run_hw_queue suspicious rcu usage
On Thu, 5 Sep 2019, Christoph Hellwig wrote:

> > Hi Christoph, Jens, and Ming,
> >
> > While booting a 5.2 SEV-enabled guest we have encountered the following
> > WARNING that is followed up by a BUG because we are in atomic context
> > while trying to call set_memory_decrypted:
>
> Well, this really is a x86 / DMA API issue unfortunately. Drivers
> are allowed to do GFP_ATOMIC dma allocation under locks / rcu critical
> sections and from interrupts. And it seems like the SEV case can't
> handle that. We have some semi-generic code to have a fixed sized
> pool in kernel/dma for non-coherent platforms that have similar issues
> that we could try to wire up, but I wonder if there is a better way
> to handle the issue, so I've added Tom and the x86 maintainers.
>
> Now independent of that issue using DMA coherent memory for the nvme
> PRPs/SGLs doesn't actually feel very optional. We could do with
> normal kmalloc allocations and just sync it to the device and back.
> I wonder if we should create some general mempool-like helpers for that.
>

Thanks for looking into this. I assume it's a non-starter to try to
address this in _vm_unmap_aliases() itself, i.e. rely on a purge spinlock
to do all synchronization (or trylock if not forced) for
purge_vmap_area_lazy() rather than only the vmap_area_lock within it. In
other words, no mutex.

If that's the case, and set_memory_encrypted() can't be fixed to not need
to sleep by changing _vm_unmap_aliases() locking, then I assume dmapool is
our only alternative? I have no idea with how large this should be.

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