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To:     Michal Hocko <mhocko@suse.com>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Subject: Re: [PATCH v17 00/10] mm: introduce memfd_secret system call to
 create "secret" memory areas
Message-ID: <8cbfe2c3-cfc6-72e0-bab1-852f80e20684@redhat.com>
Date:   Tue, 9 Feb 2021 17:17:22 +0100
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On 09.02.21 14:25, Michal Hocko wrote:
> On Tue 09-02-21 11:23:35, David Hildenbrand wrote:
> [...]
>> I am constantly trying to fight for making more stuff MOVABLE instead of
>> going into the other direction (e.g., because it's easier to implement,
>> which feels like the wrong direction).
>>
>> Maybe I am the only person that really cares about ZONE_MOVABLE these days
>> :) I can't stop such new stuff from popping up, so at least I want it to be
>> documented.
> 
> MOVABLE zone is certainly an important thing to keep working. And there
> is still quite a lot of work on the way. But as I've said this is more
> of a outlier than a norm. On the other hand movable zone is kinda hard
> requirement for a lot of application and it is to be expected that
> many features will be less than 100% compatible.  Some usecases even
> impossible. That's why I am arguing that we should have a central
> document where the movable zone is documented with all the potential
> problems we have encountered over time and explicitly state which
> features are fully/partially incompatible.
> 

I'll send a mail during the next weeks to gather current restrictions to 
document them (and include my brain dump). We might see more excessive 
use of ZONE_MOVABLE in the future and as history told us, of CMA as 
well. We really should start documenting/caring.

@Mike, it would be sufficient for me if one of your patches at least 
mention the situation in the description like

"Please note that secretmem currently behaves much more like long-term 
GUP instead of mlocked memory; secretmem is unmovable memory directly 
consumed/controlled by user space. secretmem cannot be placed onto 
ZONE_MOVABLE/CMA.

As long as there is no excessive use of secretmem (e.g., maximum of 16 
MiB for selected processes) in combination with ZONE_MOVABLE/CMA, this 
is barely a real issue. However, it is something to keep in mind when a 
significant amount of system RAM might be used for secretmem. In the 
future, we might support migration of secretmem and make it look much 
more like mlocked memory instead."

Just a suggestion.

-- 
Thanks,

David / dhildenb