Re: [PATCH v4 03/10] KVM: guest_memfd: Allow host to map guest_memfd() pages

[David Hildenbrand <david@redhat.com>]

On 20.02.25 16:45, Fuad Tabba wrote:
> Hi David,
> 
> On Thu, 20 Feb 2025 at 12:04, Fuad Tabba <tabba@google.com> wrote:
>>
>> On Thu, 20 Feb 2025 at 11:58, David Hildenbrand <david@redhat.com> wrote:
>>>
>>> On 18.02.25 18:24, Fuad Tabba wrote:
>>>> Add support for mmap() and fault() for guest_memfd backed memory
>>>> in the host for VMs that support in-place conversion between
>>>> shared and private. To that end, this patch adds the ability to
>>>> check whether the VM type supports in-place conversion, and only
>>>> allows mapping its memory if that's the case.
>>>>
>>>> This behavior is also gated by the configuration option
>>>> KVM_GMEM_SHARED_MEM.
>>>>
>>>> Signed-off-by: Fuad Tabba <tabba@google.com>
>>>> ---
>>>>    include/linux/kvm_host.h |  11 +++++
>>>>    virt/kvm/guest_memfd.c   | 103 +++++++++++++++++++++++++++++++++++++++
>>>>    2 files changed, 114 insertions(+)
>>>>
>>>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>>>> index 3ad0719bfc4f..f9e8b10a4b09 100644
>>>> --- a/include/linux/kvm_host.h
>>>> +++ b/include/linux/kvm_host.h
>>>> @@ -728,6 +728,17 @@ static inline bool kvm_arch_has_private_mem(struct kvm *kvm)
>>>>    }
>>>>    #endif
>>>>
>>>> +/*
>>>> + * Arch code must define kvm_arch_gmem_supports_shared_mem if support for
>>>> + * private memory is enabled and it supports in-place shared/private conversion.
>>>> + */
>>>> +#if !defined(kvm_arch_gmem_supports_shared_mem) && !IS_ENABLED(CONFIG_KVM_PRIVATE_MEM)
>>>> +static inline bool kvm_arch_gmem_supports_shared_mem(struct kvm *kvm)
>>>> +{
>>>> +     return false;
>>>> +}
>>>> +#endif
>>>> +
>>>>    #ifndef kvm_arch_has_readonly_mem
>>>>    static inline bool kvm_arch_has_readonly_mem(struct kvm *kvm)
>>>>    {
>>>> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
>>>> index c6f6792bec2a..30b47ff0e6d2 100644
>>>> --- a/virt/kvm/guest_memfd.c
>>>> +++ b/virt/kvm/guest_memfd.c
>>>> @@ -317,9 +317,112 @@ void kvm_gmem_handle_folio_put(struct folio *folio)
>>>>    {
>>>>        WARN_ONCE(1, "A placeholder that shouldn't trigger. Work in progress.");
>>>>    }
>>>> +
>>>> +static bool kvm_gmem_offset_is_shared(struct file *file, pgoff_t index)
>>>> +{
>>>> +     struct kvm_gmem *gmem = file->private_data;
>>>> +
>>>> +     /* For now, VMs that support shared memory share all their memory. */
>>>> +     return kvm_arch_gmem_supports_shared_mem(gmem->kvm);
>>>> +}
>>>> +
>>>> +static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
>>>> +{
>>>> +     struct inode *inode = file_inode(vmf->vma->vm_file);
>>>> +     struct folio *folio;
>>>> +     vm_fault_t ret = VM_FAULT_LOCKED;
>>>> +
>>>> +     filemap_invalidate_lock_shared(inode->i_mapping);
>>>> +
>>>> +     folio = kvm_gmem_get_folio(inode, vmf->pgoff);
>>>> +     if (IS_ERR(folio)) {
>>>> +             switch (PTR_ERR(folio)) {
>>>> +             case -EAGAIN:
>>>> +                     ret = VM_FAULT_RETRY;
>>>> +                     break;
>>>> +             case -ENOMEM:
>>>> +                     ret = VM_FAULT_OOM;
>>>> +                     break;
>>>> +             default:
>>>> +                     ret = VM_FAULT_SIGBUS;
>>>> +                     break;
>>>> +             }
>>>> +             goto out_filemap;
>>>> +     }
>>>> +
>>>> +     if (folio_test_hwpoison(folio)) {
>>>> +             ret = VM_FAULT_HWPOISON;
>>>> +             goto out_folio;
>>>> +     }
>>>> +
>>>> +     /* Must be called with folio lock held, i.e., after kvm_gmem_get_folio() */
>>>> +     if (!kvm_gmem_offset_is_shared(vmf->vma->vm_file, vmf->pgoff)) {
>>>> +             ret = VM_FAULT_SIGBUS;
>>>> +             goto out_folio;
>>>> +     }
>>>> +
>>>> +     /*
>>>> +      * Only private folios are marked as "guestmem" so far, and we never
>>>> +      * expect private folios at this point.
>>>> +      */
>>>> +     if (WARN_ON_ONCE(folio_test_guestmem(folio)))  {
>>>> +             ret = VM_FAULT_SIGBUS;
>>>> +             goto out_folio;
>>>> +     }
>>>> +
>>>> +     /* No support for huge pages. */
>>>> +     if (WARN_ON_ONCE(folio_test_large(folio))) {
>>>> +             ret = VM_FAULT_SIGBUS;
>>>> +             goto out_folio;
>>>> +     }
>>>> +
>>>> +     if (!folio_test_uptodate(folio)) {
>>>> +             clear_highpage(folio_page(folio, 0));
>>>> +             kvm_gmem_mark_prepared(folio);
>>>> +     }
>>>
>>> kvm_gmem_get_pfn()->__kvm_gmem_get_pfn() seems to call
>>> kvm_gmem_prepare_folio() instead.
>>>
>>> Could we do the same here?
>>
>> Will do.
> 
> I realized it's not that straightforward. __kvm_gmem_prepare_folio()
> requires the kvm_memory_slot, which is used to calculate the gfn. At
> that point we have neither, and it's not just an issue of access, but
> there might not be a slot associated with that yet.

Hmm, right ... I wonder if that might be problematic. I assume no 
memslot == no memory attribute telling us if it is private or shared at 
least for now?

Once guest_memfd maintains that state, it might be "cleaner" ? What's 
your thought?

-- 
Cheers,

David / dhildenb