linux-kernel - Re: [PATCH v13 04/12] KVM: guest_memfd: Add slab-allocated inode cache

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Message-ID: <343bc9ae-17d7-4466-8788-adc68acccca4@suse.cz>
Date: Mon, 27 Oct 2025 12:06:01 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Sean Christopherson <seanjc@...gle.com>, Miguel Ojeda <ojeda@...nel.org>,
 Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
 kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Ackerley Tng <ackerleytng@...gle.com>, Shivank Garg <shivankg@....com>,
 David Hildenbrand <david@...hat.com>, Fuad Tabba <tabba@...gle.com>,
 Ashish Kalra <ashish.kalra@....com>
Subject: Re: [PATCH v13 04/12] KVM: guest_memfd: Add slab-allocated inode
 cache

On 10/16/25 19:28, Sean Christopherson wrote:
> From: Shivank Garg <shivankg@....com>
> 
> Add a dedicated gmem_inode structure and a slab-allocated inode cache for
> guest memory backing, similar to how shmem handles inodes.
> 
> This adds the necessary allocation/destruction functions and prepares
> for upcoming guest_memfd NUMA policy support changes.  Using a dedicated
> structure will also allow for additional cleanups, e.g. to track flags in
> gmem_inode instead of i_private.
> 
> Signed-off-by: Shivank Garg <shivankg@....com>
> Tested-by: Ashish Kalra <ashish.kalra@....com>
> [sean: s/kvm_gmem_inode_info/gmem_inode, name init_once()]
> Reviewed-by: Ackerley Tng <ackerleytng@...gle.com>
> Tested-by: Ackerley Tng <ackerleytng@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Some nits below, not critical unless there's resubmit for other reasons:

> @@ -860,13 +917,31 @@ static int kvm_gmem_init_mount(void)
>  
>  int kvm_gmem_init(struct module *module)
>  {
> +	struct kmem_cache_args args = {
> +		.align = 0,

This seems unnecessary as it's implicit.

> +		.ctor = kvm_gmem_init_inode_once,
> +	};
> +	int ret;
> +
>  	kvm_gmem_fops.owner = module;
> +	kvm_gmem_inode_cachep = kmem_cache_create("kvm_gmem_inode_cache",
> +						  sizeof(struct gmem_inode),
> +						  &args, SLAB_ACCOUNT);
> +	if (!kvm_gmem_inode_cachep)
> +		return -ENOMEM;
>  
> -	return kvm_gmem_init_mount();
> +	ret = kvm_gmem_init_mount();
> +	if (ret) {
> +		kmem_cache_destroy(kvm_gmem_inode_cachep);
> +		return ret;
> +	}
> +	return 0;
>  }
>  
>  void kvm_gmem_exit(void)
>  {
>  	kern_unmount(kvm_gmem_mnt);
>  	kvm_gmem_mnt = NULL;
> +	rcu_barrier();

Is it because VFS can do call_rcu() with something that ends up with
kvm_gmem_free_inode()? Because nothing in this patch does that directly,
maybe worth a comment?

> +	kmem_cache_destroy(kvm_gmem_inode_cachep);
>  }