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

[Michal Hocko]

On Wed 18-03-26 12:04:10, Daniil Tatianin wrote:
> 
> On 3/18/26 11:25 AM, Michal Hocko wrote:
> > On Tue 17-03-26 23:17:28, Daniil Tatianin wrote:
> > > 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.
> > In general any attributes should have proper hieararchical semantic. I
> > am not sure what that should be in this case. What is a desire in a
> > child cgroup can become fragmentation pressure to others.
> > 
> > I think it would be really important to explain more thoroughly about
> > those usecases of mixed workloads.
> I think there are many examples of a system where one process is more
> important than
> others. For example, any sort of healthcheck or even the ssh daemon: these
> may become
> unresponsive during heavy compaction due to thousands of TLB invalidate IPIs
> or page faulting
> on pages that are being compacted. Another example is a VM that is
> responsible for routing
> traffic of all other VMs or even the entire cluster, you really want to
> prioritize its responsiveness, while
> still allowing compaction of memory for the rest of the system, for less
> important VMs or services etc.

Shouldn't those use mlock?

> > Is the memcg even a suitable level of
> > abstraction for this tunable?
> 
> In my opinion it is, since it is relatively common to put all related tasks
> into one cgroup with preset memory limits etc.
> 
> > Doesn't this belong to tasks if anything?
> 
> I think it would be very difficult to implement as a per-task attribute
> properly since compaction works at the folio
> level. While folios have a pointer to the memcg that owns them, they may be
> mapped by multiple process in case
> of shared memory. We would have to find all the address spaces mapping this
> folio, and then check the property on
> every one of them, which may be set to different values. This may be
> problematic performance-wise to do for
> every physical page, and it also introduces unclear semantics if different
> address spaces mapping the same page
> have different opinions.

Yes, it would need to be something like an implicit mlock. I haven't
really indicated that would be a _simpler_ solution. But as this has
obvious userspace API implications the much more important question is
what is a futureproof solution. Also we need to get an answer whether
this is really needed or too niche to cast an interface maintained for
ever for.
-- 
Michal Hocko
SUSE Labs