linux-kernel - Re: [PATCH v13 12/12] KVM: guest_memfd: Add gmem_inode.flags field instead of using i

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Message-ID: <45082f08-c7ec-4d45-81d8-5bce4f2fb0d8@amd.com>
Date: Fri, 17 Oct 2025 16:31:37 +0530
From: "Garg, Shivank" <shivankg@....com>
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>, David Hildenbrand <david@...hat.com>,
 Fuad Tabba <tabba@...gle.com>, Ashish Kalra <ashish.kalra@....com>,
 Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v13 12/12] KVM: guest_memfd: Add gmem_inode.flags field
 instead of using i_private



On 10/16/2025 10:58 PM, Sean Christopherson wrote:
> Track a guest_memfd instance's flags in gmem_inode instead of burying them
> in i_private.  Burning an extra 8 bytes per inode is well worth the added
> clarity provided by explicit tracking.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  virt/kvm/guest_memfd.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 4463643bd0a2..20f6e7fab58d 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -30,6 +30,8 @@ struct gmem_file {
>  struct gmem_inode {
>  	struct shared_policy policy;
>  	struct inode vfs_inode;
> +
> +	u64 flags;
>  };
>  
>  static __always_inline struct gmem_inode *GMEM_I(struct inode *inode)
> @@ -154,7 +156,7 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
>  
>  static enum kvm_gfn_range_filter kvm_gmem_get_invalidate_filter(struct inode *inode)
>  {
> -	if ((u64)inode->i_private & GUEST_MEMFD_FLAG_INIT_SHARED)
> +	if (GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_INIT_SHARED)
>  		return KVM_FILTER_SHARED;
>  
>  	return KVM_FILTER_PRIVATE;
> @@ -385,9 +387,7 @@ static inline struct file *kvm_gmem_get_file(struct kvm_memory_slot *slot)
>  
>  static bool kvm_gmem_supports_mmap(struct inode *inode)
>  {
> -	const u64 flags = (u64)inode->i_private;
> -
> -	return flags & GUEST_MEMFD_FLAG_MMAP;
> +	return GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_MMAP;
>  }
>  
>  static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
> @@ -399,7 +399,7 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
>  	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
>  		return VM_FAULT_SIGBUS;
>  
> -	if (!((u64)inode->i_private & GUEST_MEMFD_FLAG_INIT_SHARED))
> +	if (!(GMEM_I(inode)->flags & GUEST_MEMFD_FLAG_INIT_SHARED))
>  		return VM_FAULT_SIGBUS;
>  
>  	folio = kvm_gmem_get_folio(inode, vmf->pgoff);
> @@ -588,7 +588,6 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
>  		goto err_fops;
>  	}
>  
> -	inode->i_private = (void *)(unsigned long)flags;
>  	inode->i_op = &kvm_gmem_iops;
>  	inode->i_mapping->a_ops = &kvm_gmem_aops;
>  	inode->i_mode |= S_IFREG;
> @@ -598,6 +597,8 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
>  	/* Unmovable mappings are supposed to be marked unevictable as well. */
>  	WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
>  
> +	GMEM_I(inode)->flags = flags;
> +
>  	file = alloc_file_pseudo(inode, kvm_gmem_mnt, name, O_RDWR, &kvm_gmem_fops);
>  	if (IS_ERR(file)) {
>  		err = PTR_ERR(file);
> @@ -917,6 +918,8 @@ static struct inode *kvm_gmem_alloc_inode(struct super_block *sb)
>  		return NULL;
>  
>  	mpol_shared_policy_init(&gi->policy, NULL);
> +
> +	gi->flags = 0;

I initially wondered why these are initialized in kvm_gmem_alloc_inode() 
instead of kvm_gmem_init_inode_once().

Your comment in kvm_gmem_init_inode_once() made clear to me.. The ctor runs
once per slab object, so  gi->flags wouldn't reset on inode realloction.
Initializing in the alloc_inode() ensures proper reset on every allocation,
Makes sense!

Thanks for the detailed comment!

Reviewed-by: Shivank Garg <shivankg@....com>
Tested-by: Shivank Garg <shivankg@....com>

>  	return &gi->vfs_inode;
>  }
>