Re: [PATCH v5 04/13] mm/shmem: Restrict MFD_INACCESSIBLE memory against RLIMIT_MEMLOCK

[David Hildenbrand]

On 12.04.22 16:36, Jason Gunthorpe wrote:
> On Fri, Apr 08, 2022 at 08:54:02PM +0200, David Hildenbrand wrote:
> 
>> RLIMIT_MEMLOCK was the obvious candidate, but as we discovered int he
>> past already with secretmem, it's not 100% that good of a fit (unmovable
>> is worth than mlocked). But it gets the job done for now at least.
> 
> No, it doesn't. There are too many different interpretations how
> MELOCK is supposed to work
> 
> eg VFIO accounts per-process so hostile users can just fork to go past
> it.
> 
> RDMA is per-process but uses a different counter, so you can double up
> 
> iouring is per-user and users a 3rd counter, so it can triple up on
> the above two

Thanks for that summary, very helpful.

> 
>> So I'm open for alternative to limit the amount of unmovable memory we
>> might allocate for user space, and then we could convert seretmem as well.
> 
> I think it has to be cgroup based considering where we are now :\

Most probably. I think the important lessons we learned are that

* mlocked != unmovable.
* RLIMIT_MEMLOCK should most probably never have been abused for
  unmovable memory (especially, long-term pinning)


-- 
Thanks,

David / dhildenb