Re: [PATCH] KVM: SVM: Consider NUMA affinity when allocating per-CPU save_area

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On 4/23/24 14:08, Yosry Ahmed wrote:
On Tue, Apr 23, 2024 at 12:00 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
On Tue, Apr 23, 2024, Yosry Ahmed wrote:
On Tue, Apr 23, 2024 at 11:43 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
But snp_safe_alloc_page() should again flow alloc_pages() and pass numa_node_id(),

alloc_pages_node() will use numa_node_id() if you pass in NUMA_NO_NODE.

The code uses numa_mem_id()

         if (nid == NUMA_NO_NODE)
                 nid = numa_mem_id();

which is presumably the exact same thing as numa_node_id() on x86.  But I don't
want to have to think that hard :-)

Uh yes, I missed numa_node_id() vs numa_mem_id(). Anyway, using
NUMA_NO_NODE with alloc_pages_node() is intended as an abstraction
such that you don't need to think about it :P

In other words, all I'm saying is that if we want to mirror alloc_pages() and
alloc_pages_node(), then we should mirror them exactly.

It's different though, these functions are core MM APIs used by the
entire kernel. snp_safe_alloc_page() is just a helper here wrapping
those core MM APIs rather than mirroring them.

In this case snp_safe_alloc_page() would wrap
snp_safe_alloc_page_node() which would wrap alloc_pages_node(). So it
should use alloc_pages_node() as intended: pass in a node id or
NUMA_NO_NODE if you just want the closest node.

Just my 2c, not that it will make a difference in practice.

Pretty much agree with everything everyone has said. The per-CPU shouldn't be GFP_KERNEL_ACCOUNT and having a snp_safe_alloc_page() that wraps snp_safe_alloc_page_node() which then calls alloc_pages_node() seems the best way to me.


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