Re: [PATCH] mm: add memory.compact_unevictable_allowed cgroup attribute

[Daniil Tatianin]

On 3/18/26 1:01 PM, Michal Hocko wrote:
> On Wed 18-03-26 12:25:17, Daniil Tatianin wrote:
> > On 3/18/26 12:20 PM, Michal Hocko wrote:
> [...]
> > > Shouldn't those use mlock?
> > Absolutely, mlock is required to mark a folio as unevictable. Note that
> > unevictable folios are still
> > perfectly eligible for compaction. This new property makes it so a cgroup
> > can say whether its
> > unevictable pages should be compacted (same as the global
> > compact_unevictable_allowed sysctl).
> If the mlock is already used then why do we need a per memcg control as
> well? Do we have different classes of mlocked pages some with acceptable
> compaction while others without?
The way it works is mlock(2) only prevents pages from being evicted from 
the page cache by
setting unevictable | mlocked flags on the page. Such pages, however, 
are still allowed for
compaction by default, unless /proc/sys/vm/compact_unevictable_allowed 
is set to 0. That
property essentially "promotes" ALL such (unevictable) pages to a new 
synthetic tier by
making compaction skip them. The per-cgroup property works similarly, 
however, it allows
the scope to be much smaller: from a global setting that promotes 
literally ALL unevictable
(mlocked) pages to this tier, to only promoting pages belonging to the 
cgroup that has
memory.compact_unevictable_allowed as 0.