Re: [PATCH] Increase default MLOCK_LIMIT to 8 MiB

David Hildenbrand
2021-11-24T08:57:42Z

On 24.11.21 00:59, Jason Gunthorpe wrote:
> On Tue, Nov 23, 2021 at 11:04:04PM +0100, Vlastimil Babka wrote:
>> On 11/23/21 18:00, Jason Gunthorpe wrote:
>>>
>>>> believe what you say and I trust your experience :) So could as well be
>>>> that on such a "special" (or not so special) systems there should be a
>>>> way to restrict it to privileged users only.
>>>
>>> At this point RDMA is about as "special" as people running large
>>> ZONE_MOVABLE systems, and the two are going to start colliding
>>> heavily. The RDMA VFIO migration driver should be merged soon which
>>> makes VMs using this stuff finally practical.
>>
>> How does that work, I see the word migration, so does it cause pages to
>
> Sorry I mean what is often called "VM live migration". Typically that
> cannot be done if a PCI device is assigned to the VM as suspending and
> the migrating a PCI device to another server is complicated. With
> forthcoming hardware mlx5 can do this and thus the entire RDMA stack
> becomes practically usable and performant within a VM.
>
>> be migrated out of ZONE_MOVABLE before they are pinned?
>
> GUP already does this automatically for FOLL_LONGTERM.
>
>> Similarly for io-uring we could be migrating pages to be pinned so that
>> the end up consolidated close together, and prevent pathologic
>> situations like in David's reproducer.
>
> It is an interesting idea to have GUP do some kind of THP preserving
> migration.

Unfortunately it will only be a band aid AFAIU. I can rewrite my
reproducer fairly easily to pin the whole 2M range first, pin a second
time only a single page, and then unpin the 2M range, resulting in the
very same way to block THP. (I can block some THP less because I always
need the possibility to memlock 2M first, though).

--
Thanks,

David / dhildenb