Message-ID: <6c2c92eb-1699-4256-ab67-7c4dcee4e547@amd.com>
Date: Thu, 16 Oct 2025 18:28:26 +0530
From: "Garg, Shivank" <shivankg@....com>
To: Sean Christopherson <seanjc@...gle.com>, Gregory Price <gourry@...rry.net>
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Subject: Re: [f2fs-dev] [PATCH kvm-next V11 6/7] KVM: guest_memfd: Enforce
 NUMA mempolicy using shared policy



On 10/16/2025 4:18 AM, Sean Christopherson wrote:
> On Wed, Oct 15, 2025, Gregory Price wrote:
>> On Fri, Sep 26, 2025 at 12:36:27PM -0700, Sean Christopherson via Linux-f2fs-devel wrote:
>>>>
>>>> static struct mempolicy *kvm_gmem_get_policy(struct vm_area_struct *vma,
>>>> 					     unsigned long addr, pgoff_t *pgoff)
>>>> {
>>>> 	*pgoff = vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT);
>>>>
>>>> 	return __kvm_gmem_get_policy(GMEM_I(file_inode(vma->vm_file)), *pgoff);
>>>
>>> Argh!!!!!  This breaks the selftest because do_get_mempolicy() very specifically
>>> falls back to the default_policy, NOT to the current task's policy.  That is
>>> *exactly* the type of subtle detail that needs to be commented, because there's
>>> no way some random KVM developer is going to know that returning NULL here is
>>> important with respect to get_mempolicy() ABI.
>>>
>>
>> Do_get_mempolicy was designed to be accessed by the syscall, not as an
>> in-kernel ABI.
> 
> Ya, by "get_mempolicy() ABI" I meant the uABI for the get_mempolicy syscall.
> 
>> get_task_policy also returns the default policy if there's nothing
>> there, because that's what applies.
>>
>> I have dangerous questions:
> 
> Not dangerous at all, I find them very helpful!
> 
>> why is __kvm_gmem_get_policy using
>> 	mpol_shared_policy_lookup()
>> instead of
>> 	get_vma_policy()
> 
> With the disclaimer that I haven't followed the gory details of this series super
> closely, my understanding is...
> 
> Because the VMA is a means to an end, and we want the policy to persist even if
> the VMA goes away.
> 
> With guest_memfd, KVM effectively inverts the standard MMU model.  Instead of mm/
> being the primary MMU and KVM being a secondary MMU, guest_memfd is the primary
> MMU and any VMAs are secondary (mostly; it's probably more like 1a and 1b).  This
> allows KVM to map guest_memfd memory into a guest without a VMA, or with more
> permissions than are granted to host userspace, e.g. guest_memfd memory could be
> writable by the guest, but read-only for userspace.
> 
> But we still want to support things like mbind() so that userspace can ensure
> guest_memfd allocations align with the vNUMA topology presented to the guest,
> or are bound to the NUMA node where the VM will run.  We considered adding equivalent
> file-based syscalls, e.g. fbind(), but IIRC the consensus was that doing so was
> unnecessary (and potentially messy?) since we were planning on eventually adding
> mmap() support to guest_memfd anyways.
> 
>> get_vma_policy does this all for you
> 
> I assume that doesn't work if the intent is for new VMAs to pick up the existing
> policy from guest_memfd?  And more importantly, guest_memfd needs to hook
> ->set_policy so that changes through e.g. mbind() persist beyond the lifetime of
> the VMA.
> 
Additionally, the shared_policy based design enables range-based policies via its RB-tree
implementation. IIUC, this will not work with VMA-specific policy design.

>> struct mempolicy *get_vma_policy(struct vm_area_struct *vma,
>>                                  unsigned long addr, int order, pgoff_t *ilx)
>> {
>>         struct mempolicy *pol;
>>
>>         pol = __get_vma_policy(vma, addr, ilx);
>>         if (!pol)
>>                 pol = get_task_policy(current);
>>         if (pol->mode == MPOL_INTERLEAVE ||
>>             pol->mode == MPOL_WEIGHTED_INTERLEAVE) {
>>                 *ilx += vma->vm_pgoff >> order;
>>                 *ilx += (addr - vma->vm_start) >> (PAGE_SHIFT + order);
>>         }
>>         return pol;
>> }
>>
>> Of course you still have the same issue: get_task_policy will return the
>> default, because that's what applies.
>>
>> do_get_mempolicy just seems like the completely incorrect interface to
>> be using here.

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