Re: [f2fs-dev] [PATCH kvm-next V11 4/7] KVM: guest_memfd: Use guest mem inodes instead of anonymous inodes

[Garg, Shivank via Linux-f2fs-devel]

On 9/25/2025 8:20 AM, Sean Christopherson wrote:
> My apologies for the super late feedback.  None of this is critical 
> (mechanical
> things that can be cleaned up after the fact), so if there's any urgency to
> getting this series into 6.18, just ignore it.
> 
> On Wed, Aug 27, 2025, Ackerley Tng wrote:
>> Shivank Garg <shiva...@amd.com> writes:
>> @@ -463,11 +502,70 @@ bool __weak kvm_arch_supports_gmem_mmap(struct kvm 
>> *kvm)
>>      return true;
>>  }
>>
>> +static struct inode *kvm_gmem_inode_create(const char *name, loff_t size,
>> +                                       u64 flags)
>> +{
>> +    struct inode *inode;
>> +
>> +    inode = anon_inode_make_secure_inode(kvm_gmem_mnt->mnt_sb, name, NULL);
>> +    if (IS_ERR(inode))
>> +            return inode;
>> +
>> +    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;
>> +    inode->i_size = size;
>> +    mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
>> +    mapping_set_inaccessible(inode->i_mapping);
>> +    /* Unmovable mappings are supposed to be marked unevictable as well. */
>> +    WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
>> +
>> +    return inode;
>> +}
>> +
>> +static struct file *kvm_gmem_inode_create_getfile(void *priv, loff_t size,
>> +                                              u64 flags)
>> +{
>> +    static const char *name = "[kvm-gmem]";
>> +    struct inode *inode;
>> +    struct file *file;
>> +    int err;
>> +
>> +    err = -ENOENT;
>> +    /* __fput() will take care of fops_put(). */
>> +    if (!fops_get(&kvm_gmem_fops))
>> +            goto err;
>> +
>> +    inode = kvm_gmem_inode_create(name, size, flags);
>> +    if (IS_ERR(inode)) {
>> +            err = PTR_ERR(inode);
>> +            goto err_fops_put;
>> +    }
>> +
>> +    file = alloc_file_pseudo(inode, kvm_gmem_mnt, name, O_RDWR,
>> +                             &kvm_gmem_fops);
>> +    if (IS_ERR(file)) {
>> +            err = PTR_ERR(file);
>> +            goto err_put_inode;
>> +    }
>> +
>> +    file->f_flags |= O_LARGEFILE;
>> +    file->private_data = priv;
>> +
>> +    return file;
>> +
>> +err_put_inode:
>> +    iput(inode);
>> +err_fops_put:
>> +    fops_put(&kvm_gmem_fops);
>> +err:
>> +    return ERR_PTR(err);
>> +}
> 
> I don't see any reason to add two helpers.  It requires quite a bit more lines
> of code due to adding more error paths and local variables, and IMO doesn't 
> make
> the code any easier to read.
> 
> Passing in "gmem" as @priv is especially ridiculous, as it adds code and
> obfuscates what file->private_data is set to.
> 
> I get the sense that the code was written to be a "replacement" for common 
> APIs,
> but that is nonsensical (no pun intended).
> 
>>  static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
>>  {
>> -    const char *anon_name = "[kvm-gmem]";
>>      struct kvm_gmem *gmem;
>> -    struct inode *inode;
>>      struct file *file;
>>      int fd, err;
>>
>> @@ -481,32 +579,16 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t 
>> size, u64 flags)
>>              goto err_fd;
>>      }
>>
>> -    file = anon_inode_create_getfile(anon_name, &kvm_gmem_fops, gmem,
>> -                                     O_RDWR, NULL);
>> +    file = kvm_gmem_inode_create_getfile(gmem, size, flags);
>>      if (IS_ERR(file)) {
>>              err = PTR_ERR(file);
>>              goto err_gmem;
>>      }
>>
>> -    file->f_flags |= O_LARGEFILE;
>> -
>> -    inode = file->f_inode;
>> -    WARN_ON(file->f_mapping != inode->i_mapping);
>> -
>> -    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;
>> -    inode->i_size = size;
>> -    mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
>> -    mapping_set_inaccessible(inode->i_mapping);
>> -    /* Unmovable mappings are supposed to be marked unevictable as well. */
>> -    WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
>> -
>>      kvm_get_kvm(kvm);
>>      gmem->kvm = kvm;
>>      xa_init(&gmem->bindings);
>> -    list_add(&gmem->entry, &inode->i_mapping->i_private_list);
>> +    list_add(&gmem->entry, &file_inode(file)->i_mapping->i_private_list);
> 
> I don't understand this change?  Isn't file_inode(file) == inode?
> 
> Compile tested only, and again not critical, but it's -40 LoC...
> 
> 

Thanks.
I did functional testing and it works fine.


> ---
>  include/uapi/linux/magic.h |  1 +
>  virt/kvm/guest_memfd.c     | 75 ++++++++++++++++++++++++++++++++------
>  virt/kvm/kvm_main.c        |  7 +++-
>  virt/kvm/kvm_mm.h          |  9 +++--
>  4 files changed, 76 insertions(+), 16 deletions(-)
> 
> diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
> index bb575f3ab45e..638ca21b7a90 100644
> --- a/include/uapi/linux/magic.h
> +++ b/include/uapi/linux/magic.h
> @@ -103,5 +103,6 @@
>  #define DEVMEM_MAGIC         0x454d444d      /* "DMEM" */
>  #define SECRETMEM_MAGIC              0x5345434d      /* "SECM" */
>  #define PID_FS_MAGIC         0x50494446      /* "PIDF" */
> +#define GUEST_MEMFD_MAGIC    0x474d454d      /* "GMEM" */
>  
>  #endif /* __LINUX_MAGIC_H__ */
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 08a6bc7d25b6..73c9791879d5 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -1,12 +1,16 @@
>  // SPDX-License-Identifier: GPL-2.0
> +#include <linux/anon_inodes.h>
>  #include <linux/backing-dev.h>
>  #include <linux/falloc.h>
> +#include <linux/fs.h>
>  #include <linux/kvm_host.h>
> +#include <linux/pseudo_fs.h>
>  #include <linux/pagemap.h>
> -#include <linux/anon_inodes.h>
>  
>  #include "kvm_mm.h"
>  
> +static struct vfsmount *kvm_gmem_mnt;
> +
>  struct kvm_gmem {
>       struct kvm *kvm;
>       struct xarray bindings;
> @@ -385,9 +389,45 @@ static struct file_operations kvm_gmem_fops = {
>       .fallocate      = kvm_gmem_fallocate,
>  };
>  
> -void kvm_gmem_init(struct module *module)
> +static int kvm_gmem_init_fs_context(struct fs_context *fc)
> +{
> +     if (!init_pseudo(fc, GUEST_MEMFD_MAGIC))
> +             return -ENOMEM;
> +
> +     fc->s_iflags |= SB_I_NOEXEC;
> +     fc->s_iflags |= SB_I_NODEV;
> +
> +     return 0;
> +}
> +
> +static struct file_system_type kvm_gmem_fs = {
> +     .name            = "guest_memfd",
> +     .init_fs_context = kvm_gmem_init_fs_context,
> +     .kill_sb         = kill_anon_super,
> +};
> +
> +static int kvm_gmem_init_mount(void)
> +{
> +     kvm_gmem_mnt = kern_mount(&kvm_gmem_fs);
> +
> +     if (IS_ERR(kvm_gmem_mnt))
> +             return PTR_ERR(kvm_gmem_mnt);
> +
> +     kvm_gmem_mnt->mnt_flags |= MNT_NOEXEC;
> +     return 0;
> +}
> +
> +int kvm_gmem_init(struct module *module)
>  {
>       kvm_gmem_fops.owner = module;
> +
> +     return kvm_gmem_init_mount();
> +}
> +
> +void kvm_gmem_exit(void)
> +{
> +     kern_unmount(kvm_gmem_mnt);
> +     kvm_gmem_mnt = NULL;
>  }
>  
>  static int kvm_gmem_migrate_folio(struct address_space *mapping,
> @@ -465,7 +505,7 @@ bool __weak kvm_arch_supports_gmem_mmap(struct kvm *kvm)
>  
>  static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
>  {
> -     const char *anon_name = "[kvm-gmem]";
> +     static const char *name = "[kvm-gmem]";
>       struct kvm_gmem *gmem;
>       struct inode *inode;
>       struct file *file;
> @@ -481,17 +521,17 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t 
> size, u64 flags)
>               goto err_fd;
>       }
>  
> -     file = anon_inode_create_getfile(anon_name, &kvm_gmem_fops, gmem,
> -                                      O_RDWR, NULL);
> -     if (IS_ERR(file)) {
> -             err = PTR_ERR(file);
> +     /* __fput() will take care of fops_put(). */
> +     if (!fops_get(&kvm_gmem_fops)) {
> +             err = -ENOENT;
>               goto err_gmem;
>       }
>  
> -     file->f_flags |= O_LARGEFILE;
> -
> -     inode = file->f_inode;
> -     WARN_ON(file->f_mapping != inode->i_mapping);
> +     inode = anon_inode_make_secure_inode(kvm_gmem_mnt->mnt_sb, name, NULL);
> +     if (IS_ERR(inode)) {
> +             err = PTR_ERR(inode);
> +             goto err_fops;
> +     }
>  
>       inode->i_private = (void *)(unsigned long)flags;
>       inode->i_op = &kvm_gmem_iops;
> @@ -503,6 +543,15 @@ 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));
>  
> +     file = alloc_file_pseudo(inode, kvm_gmem_mnt, name, O_RDWR, 
> &kvm_gmem_fops);
> +     if (IS_ERR(file)) {
> +             err = PTR_ERR(file);
> +             goto err_inode;
> +     }
> +
> +     file->f_flags |= O_LARGEFILE;
> +     file->private_data = gmem;
> +
>       kvm_get_kvm(kvm);
>       gmem->kvm = kvm;
>       xa_init(&gmem->bindings);
> @@ -511,6 +560,10 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t 
> size, u64 flags)
>       fd_install(fd, file);
>       return fd;
>  
> +err_inode:
> +     iput(inode);
> +err_fops:
> +     fops_put(&kvm_gmem_fops);
>  err_gmem:
>       kfree(gmem);
>  err_fd:
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 18f29ef93543..301d48d6e00d 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -6489,7 +6489,9 @@ int kvm_init(unsigned vcpu_size, unsigned vcpu_align, 
> struct module *module)
>       if (WARN_ON_ONCE(r))
>               goto err_vfio;
>  
> -     kvm_gmem_init(module);
> +     r = kvm_gmem_init(module);
> +     if (r)
> +             goto err_gmem;
>  
>       r = kvm_init_virtualization();
>       if (r)
> @@ -6510,6 +6512,8 @@ int kvm_init(unsigned vcpu_size, unsigned vcpu_align, 
> struct module *module)
>  err_register:
>       kvm_uninit_virtualization();
>  err_virt:
> +     kvm_gmem_exit();
> +err_gmem:
>       kvm_vfio_ops_exit();
>  err_vfio:
>       kvm_async_pf_deinit();
> @@ -6541,6 +6545,7 @@ void kvm_exit(void)
>       for_each_possible_cpu(cpu)
>               free_cpumask_var(per_cpu(cpu_kick_mask, cpu));
>       kmem_cache_destroy(kvm_vcpu_cache);
> +     kvm_gmem_exit();
>       kvm_vfio_ops_exit();
>       kvm_async_pf_deinit();
>       kvm_irqfd_exit();
> diff --git a/virt/kvm/kvm_mm.h b/virt/kvm/kvm_mm.h
> index 31defb08ccba..9fcc5d5b7f8d 100644
> --- a/virt/kvm/kvm_mm.h
> +++ b/virt/kvm/kvm_mm.h
> @@ -68,17 +68,18 @@ static inline void 
> gfn_to_pfn_cache_invalidate_start(struct kvm *kvm,
>  #endif /* HAVE_KVM_PFNCACHE */
>  
>  #ifdef CONFIG_KVM_GUEST_MEMFD
> -void kvm_gmem_init(struct module *module);
> +int kvm_gmem_init(struct module *module);
> +void kvm_gmem_exit(void);
>  int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args);
>  int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
>                 unsigned int fd, loff_t offset);
>  void kvm_gmem_unbind(struct kvm_memory_slot *slot);
>  #else
> -static inline void kvm_gmem_init(struct module *module)
> +static inline int kvm_gmem_init(struct module *module)
>  {
> -
> +     return 0;
>  }
> -
> +static inline void kvm_gmem_exit(void) {};
>  static inline int kvm_gmem_bind(struct kvm *kvm,
>                                        struct kvm_memory_slot *slot,
>                                        unsigned int fd, loff_t offset)
> 
> base-commit: d133892dddd6607de651b7e32510359a6af97c4c
> --


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