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@google.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@amd.com>
Tested-by: Shivank Garg <shivankg@amd.com>
> return &gi->vfs_inode;
> }
>