Re: [RFC PATCH v2 06/10] kvm: guest_memfd: Add support for freezing and unfreezing mappings

[Pratyush Yadav <pratyush@kernel.org>]

On Tue, Jun 23 2026, tarunsahu@google.com wrote:

> Pratyush Yadav <pratyush@kernel.org> writes:
>
>> On Tue, Jun 23 2026, tarunsahu@google.com wrote:
>>
>>> Ackerley Tng <ackerleytng@google.com> writes:
>>>
>>>> Tarun Sahu <tarunsahu@google.com> writes:
>>>>
>>>>>  static long kvm_gmem_fallocate(struct file *file, int mode, loff_t offset,
>>>>>  			       loff_t len)
>>>>>  {
>>>>> +	struct inode *inode = file_inode(file);
>>>>>  	int ret;
>>>>> +	int idx;
>>>>>
>>>>> -	if (!(mode & FALLOC_FL_KEEP_SIZE))
>>>>> -		return -EOPNOTSUPP;
>>>>> +	idx = srcu_read_lock(&kvm_gmem_freeze_srcu);
>>>>> +	if (kvm_gmem_is_frozen(inode)) {
>>>>> +		srcu_read_unlock(&kvm_gmem_freeze_srcu, idx);
>>>>> +		return -EPERM;
>>>>> +	}
>>>>
>>>> fallocate may eventually go to kvm_gmem_get_folio(), so that would check
>>>> kvm_gmem_is_frozen() twice. Is this meant to catch the punch hole case?
>>
>> Yeah, I reckon you can get away with doing this check only in
>> kvm_gmem_get_folio(). Normally you'd like to fail early, but as of now I
>> don't see much of a problem. If you drop the check here and fail in
>> kvm_gmem_get_folio() you'd end up taking and releasing the mapping
>> invalidate_lock, but this isn't a fast path anyway so I don't think it
>> should matter much.
>
> No, Don't agree.
> kvm_gmem_get_folios already have the is_frozen check. which blocks the
> kvm_gmem_allocate. But not kvm_gmem_punch_hole. Your argument is correct
> for kvm_gmem_allocate only. So is_frozen check in fallocate is to
> block the punch hole as well. What ackerley said is correct.

Oh, right. Then we do need the check in both places.

[...]

-- 
Regards,
Pratyush Yadav