[Virtio-fs] Status of DAX for virtio-fs/virtiofsd?

[Hao Xu]

On 9/6/23 21:57, Stefan Hajnoczi wrote:
> On Wed, 6 Sept 2023 at 09:07, Hao Xu <hao.xu at linux.dev> wrote:
>> On 5/18/23 00:26, Stefan Hajnoczi wrote:
>>> On Wed, 17 May 2023 at 11:54, Alex Bennée <alex.bennee at linaro.org> wrote:
>>> Hi Alex,
>>> There were two unresolved issues:
>>>
>>> 1. How to inject SIGBUS when the guest accesses a page that's beyond
>>> the end-of-file.
>> Hi Stefan,
>> Does this SIGBUS issue exist if the guest kernel can be trusted? Since in
>>
>> that case, we can check the offset value in guest kernel.
> The scenario is:
> 1. A guest userspace process has a DAX file mmapped.
> 2. The host or another guest that is also sharing the directory
> truncates the file. The pages mmapped by our guest are no longer
> valid.
> 3. The guest loads from an mmapped page and a vmexit occurs.
> 4. Now the host must inject a SIGBUS into the guest. There is
> currently no way to do this.
>
> I believe this scenario doesn't happen within a single guest, because
> the guest kernel will raise SIGBUS itself without a vmexit if another
> process inside that same guest truncates the file.
>
> Another scenario is when the guest kernel access the DAX pages. A
> vmexit can occur here too.
>
> If you trust the host and all guests sharing the directory not to
> truncate files that are mmapped, then this issue will not occur.
>
> Stefan


I see, my use case should be fine since the directory is not shared and 
fs is read-only.

Thanks for detail explanation.


Regards,

Hao