Re: [bug] __blk_mq_run_hw_queue suspicious rcu usage

[David Rientjes]

On Thu, 12 Dec 2019, David Rientjes wrote:

> Since all DMA must be unencrypted in this case, what happens if all 
> dma_direct_alloc_pages() calls go through the DMA pool in 
> kernel/dma/remap.c when force_dma_unencrypted(dev) == true since 
> __PAGE_ENC is cleared for these ptes?  (Ignoring for a moment that this 
> special pool should likely be a separate dma pool.)
> 
> I assume a general depletion of that atomic pool so 
> DEFAULT_DMA_COHERENT_POOL_SIZE becomes insufficient.  I'm not sure what 
> size any DMA pool wired up for this specific purpose would need to be 
> sized at, so I assume dynamic resizing is required.
> 
> It shouldn't be *that* difficult to supplement kernel/dma/remap.c with the 
> ability to do background expansion of the atomic pool when nearing its 
> capacity for this purpose?  I imagine that if we just can't allocate pages 
> within the DMA mask that it's the only blocker to dynamic expansion and we 
> don't oom kill for lowmem.  But perhaps vm.lowmem_reserve_ratio is good 
> enough protection?
> 
> Beyond that, I'm not sure what sizing would be appropriate if this is to 
> be a generic solution in the DMA API for all devices that may require 
> unecrypted memory.
> 

Optimizations involving lowmem reserve ratio aside, is it ok that 
CONFIG_AMD_MEM_ENCRYPT develops a dependency on DMA_DIRECT_REMAP because 
set_memory_decrypted() must be allowed to block?

If so, we could allocate from the atomic pool when we can't block and the 
device requires unencrypted DMA from dma_direct_alloc_pages().  I assume 
we need this to be its own atomic pool specifically for 
force_dma_unencrypted() devices and to check addr_in_gen_pool() for this 
new unencrypted pool in dma_direct_free_pages().

I have no idea how large this unencrypted atomic pool should be sized.  We 
could determine a nice default and grow size for nvme itself, but as 
Christoph mentioned many drivers require non-blockable allocations that 
can be run inside a SEV encrypted guest.

Trivial implementation would be to just double the size of the unencrypted 
pool when it reaches half capacity.  Perhaps done with GFP_KERNEL | 
__GFP_DMA allocations in a workqueue.  We can reclaim from ZONE_DMA or 
ZONE_DMA32 in this context but when that fails I'm not sure if it's 
satisfactory to just fail the dma_pool_alloc() when the unecrypted pool 
runs out.

Heuristics can be tweaked, of course, but I want to make sure I'm not 
missing anything obvious with this approach before implementing it.  
Please let me know, thanks.