Re: [PATCH v6 4/5] mm/migrate: skip migrating folios under writeback with AS_WRITEBACK_INDETERMINATE mappings

[Bernd Schubert <bernd.schubert@fastmail.fm>]

On 12/26/24 21:13, Shakeel Butt wrote:
> On Tue, Dec 24, 2024 at 01:37:49PM +0100, David Hildenbrand wrote:
>> On 23.12.24 23:14, Shakeel Butt wrote:
>>> On Sat, Dec 21, 2024 at 05:18:20PM +0100, David Hildenbrand wrote:
>>>>
>>>
>>> I think you have valid concerns but these are not new and not unique to
>>> fuse. Any filesystem with a potential arbitrary stall can have similar
>>> issues. The arbitrary stall can be caused due to network issues or some
>>> faultly local storage.
>>
>> What concerns me more is that this is can be triggered by even unprivileged
>> user space, and that there is no default protection as far as I understood,
>> because timeouts cannot be set universally to a sane defaults.
>>
>> Again, please correct me if I got that wrong.
>>
> 
> Let's route this question to FUSE folks. More specifically: can an
> unprivileged process create a mount point backed by itself, create a
> lot of dirty (bound by cgroup) and writeback pages on it and let the
> writeback pages in that state forever?

libfuse provides 'fusermount' which has the s-bit set. I think most 
distributions take that over into their libfuse packages. 
The fuse-server process then continues to run as arbitrary user.