Messages in this thread | | | Subject | Re: [PATCH RFC 0/2] dma-pool: allow user to disable atomic pool | From | Robin Murphy <> | Date | Thu, 24 Jun 2021 11:47:31 +0100 |
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On 2021-06-24 10:29, Baoquan He wrote: > On 06/24/21 at 08:40am, Christoph Hellwig wrote: >> So reduce the amount allocated. But the pool is needed for proper >> operation on systems with memory encryption. And please add the right >> maintainer or at least mailing list for the code you're touching next >> time. > > Oh, I thoutht it's memory issue only, should have run > ./scripts/get_maintainer.pl. sorry. > > About reducing the amount allocated, it may not help. Because on x86_64, > kdump kernel doesn't put any page of memory into buddy allocator of DMA > zone. Means it will defenitely OOM for atomic_pool_dma initialization. > > Wondering in which case or on which device the atomic pool is needed on > AMD system with mem encrytion enabled. As we can see, the OOM will > happen too in kdump kernel on Intel system, even though it's not > necessary.
Hmm, I think the Kconfig reshuffle has actually left a slight wrinkle here. For DMA_DIRECT_REMAP=y we can assume an atomic pool is always needed, since that was the original behaviour anyway. However the implications of AMD_MEM_ENCRYPT=y are different - even if support is enabled, it still should only be relevant if mem_encrypt_active(), so it probably does make sense to have an additional runtime gate on that.
From a quick scan, use of dma_alloc_from_pool() already depends on force_dma_unencrypted() so that's probably fine already, but I think we'd need a bit of extra protection around dma_free_from_pool() to prevent gen_pool_has_addr() dereferencing NULL if the pools are uninitialised, even with your proposed patch as it is. Presumably nothing actually called dma_direct_free() when you tested this?
Robin.
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