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Message-ID: <diqzbjs5k9ms.fsf@ackerleytng-ctop.c.googlers.com>
Date: Tue, 06 May 2025 09:05:15 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: pbonzini@...hat.com, tglx@...utronix.de, x86@...nel.org,
kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-kselftest@...r.kernel.org, mingo@...hat.com, bp@...en8.de,
dave.hansen@...ux.intel.com, hpa@...or.com, shuah@...nel.org,
andrew.jones@...ux.dev, ricarkol@...gle.com, chao.p.peng@...ux.intel.com,
tabba@...gle.com, jarkko@...nel.org, yu.c.zhang@...ux.intel.com,
vannapurve@...gle.com, erdemaktas@...gle.com, mail@...iej.szmigiero.name,
vbabka@...e.cz, david@...hat.com, qperret@...gle.com, michael.roth@....com,
wei.w.wang@...el.com, liam.merwick@...cle.com, isaku.yamahata@...il.com,
kirill.shutemov@...ux.intel.com, Ryan Afranji <afranji@...gle.com>
Subject: Re: [RFC PATCH 02/11] KVM: guest_mem: Add ioctl KVM_LINK_GUEST_MEMFD
Sean Christopherson <seanjc@...gle.com> writes:
> On Mon, Aug 07, 2023, Ackerley Tng wrote:
>> KVM_LINK_GUEST_MEMFD will link a gmem fd's underlying inode to a new
>> file (and fd).
>>
>> Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>
>> ---
>> include/uapi/linux/kvm.h | 8 +++++
>> virt/kvm/guest_mem.c | 73 ++++++++++++++++++++++++++++++++++++++++
>> virt/kvm/kvm_main.c | 10 ++++++
>> virt/kvm/kvm_mm.h | 7 ++++
>> 4 files changed, 98 insertions(+)
>>
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index eb900344a054..d0e2a2ce0df2 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -2299,4 +2299,12 @@ struct kvm_create_guest_memfd {
>> __u64 reserved[6];
>> };
>>
>> +#define KVM_LINK_GUEST_MEMFD _IOWR(KVMIO, 0xd5, struct kvm_link_guest_memfd)
>> +
>> +struct kvm_link_guest_memfd {
>> + __u64 fd;
>> + __u64 flags;
>> + __u64 reserved[6];
>> +};
>> +
>> #endif /* __LINUX_KVM_H */
>> diff --git a/virt/kvm/guest_mem.c b/virt/kvm/guest_mem.c
>> index 30d0ab8745ee..1b3df273f785 100644
>> --- a/virt/kvm/guest_mem.c
>> +++ b/virt/kvm/guest_mem.c
>> @@ -477,6 +477,79 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
>> return __kvm_gmem_create(kvm, size, flags, kvm_gmem_mnt);
>> }
>>
>> +static inline void __kvm_gmem_do_link(struct inode *inode)
>> +{
>> + /* Refer to simple_link() */
>> +
>> + inode->i_ctime = current_time(inode);
>> + inc_nlink(inode);
>> +
>> + /*
>> + * ihold() to add additional reference to inode for reference in dentry,
>> + * created in kvm_gmem_alloc_file() -> alloc_file_pseudo(). This is not
>> + * necessary when creating a new file because alloc_inode() creates
>> + * inodes with i_count = 1, which is the refcount for the dentry in the
>> + * file.
>> + */
>> + ihold(inode);
>> +
>> + /*
>> + * dget() and d_instantiate() complete the setup of a dentry, but those
>> + * have already been done in kvm_gmem_alloc_file() ->
>> + * alloc_file_pseudo()
>> + */
>> +}
Thanks Sean, we're just circling back to this series, working on a next
revision.
>
> Does this have to be done before the fd is exposed to userspace, or can it be
> done after?
Does "exposed to userspace" mean the call to get_unused_fd_flags(),
where an fd is reserved?
Do you mean to make this reservation as late as possible?
> If it can be done after, I'd prefer to have the allocation helper
> also install the fd, and also rename it to something that better conveys that
> it's allocating more than just the file, e.g. that it allocates and initialize
> kvm_gmem too.
>
> Completely untested, but this is what I'm thinkin/hoping.
>
> static int kvm_gmem_alloc_view(struct kvm *kvm, struct inode *inode,
> struct vfsmount *mnt)
Will rename this kvm_gmem_alloc_view(), that naming totally makes
sense, and attaches a meaning to the struct file as a view into the
memory.
> {
> struct file *file;
> struct kvm_gmem *gmem;
>
> gmem = kzalloc(sizeof(*gmem), GFP_KERNEL);
> if (!gmem)
> return -ENOMEM;
>
> fd = get_unused_fd_flags(0);
> if (fd < 0) {
> r = fd;
> goto err_fd;
> }
Do you see the fd as part of the view? I thought the fd is just a handle
to the view (struct file).
>
> file = alloc_file_pseudo(inode, mnt, "kvm-gmem", O_RDWR, &kvm_gmem_fops);
> if (IS_ERR(file)) {
> r = PTR_ERR(file);
> goto err_file;
> }
>
> file->f_flags |= O_LARGEFILE;
> file->f_mapping = inode->i_mapping;
>
> kvm_get_kvm(kvm);
> gmem->kvm = kvm;
> xa_init(&gmem->bindings);
>
> file->private_data = gmem;
>
> list_add(&gmem->entry, &inode->i_mapping->private_list);
>
> fd_install(fd, file);
>
> return 0;
> err:
> put_unused_fd(fd);
> err_fd:
> kfree(gmem);
> return r;
> }
>
> static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags,
> struct vfsmount *mnt)
> {
> const char *anon_name = "[kvm-gmem]";
> const struct qstr qname = QSTR_INIT(anon_name, strlen(anon_name));
> struct inode *inode;
> struct file *file;
> int fd, err;
>
> inode = alloc_anon_inode(mnt->mnt_sb);
> if (IS_ERR(inode))
> return PTR_ERR(inode);
>
> err = security_inode_init_security_anon(inode, &qname, NULL);
> if (err)
> goto err;
>
> 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_large_folios(inode->i_mapping);
> mapping_set_unevictable(inode->i_mapping);
> mapping_set_unmovable(inode->i_mapping);
>
> fd = kvm_gmem_alloc_view(kvm, inode, mnt);
> if (fd < 0) {
> err = fd;
> goto err;
> }
> return fd;
> err:
> iput(inode);
> return err;
> }
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