[PATCH v16 07/11] secretmem: use PMD-size pages to amortize direct map fragmentation

[David Hildenbrand]

On 02.02.21 14:32, Michal Hocko wrote:
> On Tue 02-02-21 14:14:09, David Hildenbrand wrote:
> [...]
>> As already expressed, I dislike allowing user space to consume an unlimited
>> number unmovable/unmigratable allocations. We already have that in some
>> cases with huge pages (when the arch does not support migration) - but there
>> we can at least manage the consumption using the whole max/reserved/free/...
>> infrastructure. In addition, adding arch support for migration shouldn't be
>> too complicated.
> 
> Well, mlock is not too different here as well. Hugepages are arguably an
> easier model because it requires an explicit pre-configuration by an
> admin. Mlock doesn't have anything like that. Please also note that
> while mlock pages are migrateable by default, this is not the case in
> general because they can be configured to disalow migration to prevent
> from minor page faults as some workloads require that (e.g. RT).

Yeah, however that is a very special case. In most cases mlock() simply 
prevents swapping, you still have movable pages you can place anywhere 
you like (including on ZONE_MOVABLE).

> Another example is ramdisk or even tmpfs (with swap storage depleted or
> not configured). Both are PITA from the OOM POV but they are manageable
> if people are careful.

Right, but again, special cases - e.g., tmpfs explicitly has to be resized.

> If secretmem behaves along those existing models
> then we know what to expect at least.

I think secretmem behaves much more like longterm GUP right now 
("unmigratable", "lifetime controlled by user space", "cannot go on 
CMA/ZONE_MOVABLE"). I'd either want to reasonably well control/limit it 
or make it behave more like mlocked pages.

-- 
Thanks,

David / dhildenb