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

[Daniil Tatianin]

On 3/17/26 10:17 PM, Andrew Morton wrote:
> On Tue, 17 Mar 2026 13:00:58 +0300 Daniil Tatianin <d-tatianin@xxxxxxxxxxxxxx> wrote:
>
> > The current global sysctl compact_unevictable_allowed is too coarse.
> > In environments with mixed workloads, we may want to protect specific
> > important cgroups from compaction to ensure their stability and
> > responsiveness, while allowing compaction for others.
> >
> > This patch introduces a per-memcg compact_unevictable_allowed attribute.
> > This allows granular control over whether unevictable pages in a specific
> > cgroup can be compacted. The global sysctl still takes precedence if set
> > to disallow compaction, but this new setting allows opting out specific
> > cgroups.
> >
> > This also adds a new ISOLATE_UNEVICTABLE_CHECK_MEMCG flag to
> > isolate_migratepages_block to preserve the old behavior for the
> > ISOLATE_UNEVICTABLE flag unconditionally used by
> > isolage_migratepages_range.
> AI review asked questions:
> 	https://sashiko.dev/#/patchset/20260317100058.2316997-1-d-tatianin@xxxxxxxxxxxxxx

> Should this dynamically walk up the ancestor chain during evaluation to
> ensure it returns false if any ancestor has disallowed compaction?

I think ultimately it's up to cgroup maintainers whether the code should 
do that, but as far as I understand the whole point of cgroups is that a 
child can override the settings of its parent. Moreover, this property 
doesn't have CFTYPE_NS_DELEGATABLE set, so a child cgroup cannot just 
toggle it at will.