On 8/5/21 1:54 AM, Baoquan He wrote:

On 06/24/21 at 11:47am, Robin Murphy wrote:

quoted

On 2021-06-24 10:29, Baoquan He wrote:

quoted

On 06/24/21 at 08:40am, Christoph Hellwig wrote:

quoted

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.

Sorry for very late response, and thank both for your comments.

quoted

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.

quoted

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?

Yes, enforcing the conditional check of force_dma_unencrypted() around
dma_free_from_pool sounds reasonable, just as we have done in
dma_alloc_from_pool().

I have tested this patchset on normal x86_64 systems and one amd system
with SME support, disabling atomic pool can fix the issue that there's no
managed pages in dma zone then requesting page from dma zone will cause
allocation failure. And even disabling atomic pool in 1st kernel didn't
cause any problem on one AMD EPYC system which supports SME. I am not
expert of DMA area, wondering how atomic pool is supposed to do in
SME/SEV system. 

I think the atomic pool is used by the NVMe driver. My understanding is
that driver will do a dma_alloc_coherent() from interrupt context, so it
needs to use GFP_ATOMIC. The pool was created because dma_alloc_coherent()
would perform a set_memory_decrypted() call, which can sleep. The pool
eliminates that issue (David can correct me if I got that wrong).

Thanks,
Tom

Besides, even though atomic pool is disabled, slub page for allocation
of dma-kmalloc also triggers page allocation failure. So I change to
take another way to fix them, please check v2 post. The atomic pool
disabling an be a good to have change.

Thanks
Baoquan

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