Re: [PATCH] cgroup/dmem: accept only one region per limit write

[Maxime Ripard <mripard@kernel.org>]

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On Sat, Jun 06, 2026 at 11:44:10PM +0200, Maarten Lankhorst wrote:
> Hey,
> 
> On 6/6/26 18:31, Natalie Vock wrote:
> > On 6/6/26 00:44, Eric Chanudet wrote:
> >> Accept only one "region value" pair entry for the dmem.max, dmem.min,
> >> dmem.low files.>
> >> This changes the UAPI that otherwise accepted multiple lines for setting
> >> multiple entries in one write. No existing user is known to rely on
> >> writing multiple regions in a single write.
> > 
> > Ugh, shoot.
> > 
> > For dmem.low specifically, there already are some userspace thingies floating around that may write more than one region/value pairs.
> > 
> > These thingies all depend on that one patchset for dmemcg protection that I should really get around to merging[1]. Since the userspace utilities depend on not-yet-merged patches, they sort of have to expect stuff changing under their belts, so I wouldn't really consider those users a blocker by necessity.
> > 
> > As I see it, we could go down one of two paths:
> > 1. We go ahead with the patch as proposed, and I make sure that the users I know of adapt. Could be a bit icky wrt. "do not break userspace" rules, but since the already use non-merged UAPIs in one place, you can argue that these users kind of have to expect breakage.
> > 2. We use the old handling allowing multiple lines for dmem.min and dmem.low only. This preserves compatibility but uglifies the code by quite a bit.
> > 
> > All things considered, I think I personally would prefer going with 1. and taking the patch as proposed and just having one codepath handling every limit file. Just highlighting this so we don't do it on accident.
> > 
> > [1] https://patchwork.freedesktop.org/series/163183/
> > 
> 
> I prefer option 1 as well, but would like an ack from one of the core cgroup maintainers too,
> and what Maxime's opinion on this as well.

Option 1 works for me too if doable

Maxime

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