Re: Memory compaction and mlockall()

[Vlastimil Babka <vbabka@suse.cz>]

On 7/11/19 11:43 AM, Sebastian Andrzej Siewior wrote:
> On 2019-07-10 11:21:19 [-0700], Yang Shi wrote:
>>
>> compaction should not isolate unevictable pages unless you have
>> /proc/sys/vm/compact_unevictable_allowed set.
> 
> Thank you. This is enabled by default. The documentation for this says
> | … compaction is allowed to examine the unevictable lru (mlocked pages) for
> | pages to compact.…
> 
> so it is actually clear once you know where to look.
> If I read this correct, the default behavior was to ignore mlock()ed
> pages for compaction then commit
>   5bbe3547aa3ba ("mm: allow compaction of unevictable pages")
> 
> came along in v4.1-rc1 and changed that behaviour. Is it too late to
> flip it back?

I would say that enabled is a better default wrt benefits for the
majority of systems. This was assuming that mlock() is primarily used to
prevent sensitive data (crypto keys) from hitting swap, not to give
latency guarantees. You could perhaps argue that enabling PREEMPT_RT
might change the default, but it's somewhat subtle.

> Sebastian
>