linux-kernel - Re: [f2fs-dev] [PATCH kvm-next V11 6/7] KVM: guest_memfd: Enforce NUMA mempolicy using shared policy

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Message-ID: <aPAWFQyFLK4EKWVK@gourry-fedora-PF4VCD3F>
Date: Wed, 15 Oct 2025 17:45:57 -0400
From: Gregory Price <gourry@...rry.net>
To: Sean Christopherson <seanjc@...gle.com>
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Subject: Re: [f2fs-dev] [PATCH kvm-next V11 6/7] KVM: guest_memfd: Enforce
 NUMA mempolicy using shared policy

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.

get_task_policy also returns the default policy if there's nothing
there, because that's what applies.

I have dangerous questions:

why is __kvm_gmem_get_policy using
	mpol_shared_policy_lookup()
instead of
	get_vma_policy()

get_vma_policy does this all for you

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.

~Gregory