Re: [PATCH v2 2/2] cgroup/dmem: add dmem.memcg control file for double-charging to memcg

[Eric Chanudet <echanude@redhat.com>]

On Fri, May 22, 2026 at 05:26:16PM +0200, Michal Koutný wrote:
> Hello Eric.
> 
> On Tue, May 19, 2026 at 11:59:02AM -0400, Eric Chanudet <echanude@redhat.com> wrote:
> > Add a root-only cgroupfs file "dmem.memcg" that lets an administrator
> > configure whether allocations in a dmem region should also be charged to
> > the memory controller.
> 
> This kinda makes sense as it is not unlike io.cost.* device
> configurators.
> 
> Just for my better understanding -- will there be a space for userspace
> to switch this? (No charged dmem allocations happen before responsible
> userspace runs, so that the attribute remains unlocked.)
> 
> (I'm rather indifferent about the actual double charging/non-charging
> matter.)

Yes, this is intended to be configured before the user space stack that
would start allocating things is started. Once it has started (and tried
to charge something), the configuration is locked

> 
> > 
> > To handle inheritance, dmem adds a depends_on the memory controller,
> > unless MEMCG isn't configured in.
> > 
> > Double-charging is disabled by default. Once a charge is attempted, the
> > setting is locked to prevent inconsistent accounting by a small 4-state
> > machine (off, on, locked off, locked on).
> > 
> > The memcg to charge is derived from the pool's cgroup, since the pool
> > holds a reference to the dmem cgroup state that keeps the cgroup alive
> > until it gets uncharged.
> > 
> > Signed-off-by: Eric Chanudet <echanude@redhat.com>
> > ---
> >  Documentation/admin-guide/cgroup-v2.rst |  23 +++++
> >  kernel/cgroup/dmem.c                    | 158 +++++++++++++++++++++++++++++++-
> >  2 files changed, 178 insertions(+), 3 deletions(-)
> > 
> > diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> > index 6efd0095ed995b1550317662bc1b56c7a7f3db23..1d2fa55ddf0faa17baa916a8914d3033e8e42359 100644
> > --- a/Documentation/admin-guide/cgroup-v2.rst
> > +++ b/Documentation/admin-guide/cgroup-v2.rst
> > @@ -2828,6 +2828,29 @@ DMEM Interface Files
> >  	  drm/0000:03:00.0/vram0 12550144
> >  	  drm/0000:03:00.0/stolen 8650752
> >  
> > +  dmem.memcg
> > +	A readwrite nested-keyed file that exists only on the root
> > +	cgroup.
> 
> Strictly speaking this is not nested-keyed but flat keyed [1],

Indeed,

> which leads me to realization that this is the first instance of a boolean.
> All in call, such a composition comes to my mind (latter is RO):
> 
> 	drm/0000:03:00.0/vram0 enable=0|1 locked=0|1
> 

So per[1] 1 key, 2 sub-keys (enable RW, locked RO), that looks better
and match the documentation, thanks!

> 
> 
> > +static ssize_t dmem_cgroup_memcg_write(struct kernfs_open_file *of, char *buf,
> > +				       size_t nbytes, loff_t off)
> > +{
> > +	while (buf) {
> > +		struct dmem_cgroup_region *region;
> > +		char *options, *name;
> > +		bool flag;
> > +
> > +		options = buf;
> > +		buf = strchr(buf, '\n');
> > +		if (buf)
> > +			*buf++ = '\0';
> 
> I recall there was a discussion about accepting only a single device per
> write(2) (at the same time I see this idiom is still present in other
> dmem.* files, so this is nothing to change in _this_ patch).

I would second that. When setting say dmem.max for 2 regions, with a
typo on the second, the first one is set, but write still get EINVAL.

Also, I just notice dmemcg_limit_write() returns EINVAL if the region is
not found (this patch returns ENODEV).

> 
> Thanks,
> Michal
> 
> [1] https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html#format



-- 
Eric Chanudet