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

From: David Hildenbrand <david@redhat.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Sean Christopherson <seanjc@google.com>,
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	Dave Hansen <dave.hansen@intel.com>,
	Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH v5 04/13] mm/shmem: Restrict MFD_INACCESSIBLE memory against RLIMIT_MEMLOCK
Date: Wed, 13 Apr 2022 18:24:56 +0200	[thread overview]
Message-ID: <1686fd2d-d9c3-ec12-32df-8c4c5ae26b08@redhat.com> (raw)
In-Reply-To: <20220412143636.GG64706@ziepe.ca>

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