Re: [PATCH 0/3] cgroup: nmi safe css_rstat_updated

[Shakeel Butt]

On Tue, Jun 10, 2025 at 12:53:11PM +0200, Michal Koutný wrote:
> On Mon, Jun 09, 2025 at 03:56:08PM -0700, Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
> > BPF programs can run in nmi context and may trigger memcg charged memory
> > allocation in such context. Recently linux added support to nmi safe
> > page allocation along with memcg charging of such allocations. However
> > the kmalloc/slab support and corresponding memcg charging is still
> > lacking,
> > 
> > To provide nmi safe support for memcg charging for kmalloc/slab
> > allocations, we need nmi safe memcg stats and for that we need nmi safe
> > css_rstat_updated() which adds the given cgroup state whose stats are
> > updated into the per-cpu per-ss update tree. This series took the aim to
> > make css_rstat_updated() nmi safe.
> 
> memcg charging relies on page counters and per-cpu stocks.
> css_rstat_updated() is "only" for statistics (which has admiteddly some
> in-kernel consumers but those are already affected by batching and
> flushing errors).
> 
> Have I missed some updates that make css_rstat_updated() calls critical
> for memcg charging? I'd find it useful to explain this aspect more in
> the cover letter.

For kernel memory, the charging and stats (MEMCG_KMEM,
NR_SLAB_RECLAIMABLE_B, NR_SLAB_UNRECLAIMABLE_B) updates happen together.
I will add a line or two in the next version.