linux-kernel - Re: [PATCH RFC 0/2] dma-pool: allow user to disable atomic pool

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Date:   Thu, 24 Jun 2021 11:47:31 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Baoquan He <bhe@...hat.com>, Christoph Hellwig <hch@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        rientjes@...gle.com, rppt@...ux.ibm.com, thomas.lendacky@....com,
        brijesh.singh@....com, kexec@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, m.szyprowski@...sung.com
Subject: Re: [PATCH RFC 0/2] dma-pool: allow user to disable atomic pool

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.