On Wed, Mar 1, 2023 at 12:07 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > On Wed, Mar 01, 2023 at 11:34:03AM -0800, Suren Baghdasaryan wrote: > > Current 500ms min window size for psi triggers limits polling interval > > to 50ms to prevent polling threads from using too much cpu bandwidth by > > polling too frequently. However the number of cgroups with triggers is > > unlimited, so this protection can be defeated by creating multiple > > cgroups with psi triggers (triggers in each cgroup are served by a single > > "psimon" kernel thread). > > Instead of limiting min polling period, which also limits the latency of > > psi events, it's better to limit psi trigger creation to authorized users > > only, like we do for system-wide psi triggers (/proc/pressure/* files can > > be written only by processes with CAP_SYS_RESOURCE capability). This also > > makes access rules for cgroup psi files consistent with system-wide ones. > > Add a CAP_SYS_RESOURCE capability check for cgroup psi file writers and > > remove the psi window min size limitation. > > > > Suggested-by: Sudarshan Rajagopalan <quic_sudaraja@xxxxxxxxxxx> > > Link: https://lore.kernel.org/all/cover.1676067791.git.quic_sudaraja@xxxxxxxxxxx/ > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > > --- > > kernel/cgroup/cgroup.c | 10 ++++++++++ > > kernel/sched/psi.c | 4 +--- > > 2 files changed, 11 insertions(+), 3 deletions(-) > > > > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c > > index 935e8121b21e..b600a6baaeca 100644 > > --- a/kernel/cgroup/cgroup.c > > +++ b/kernel/cgroup/cgroup.c > > @@ -3867,6 +3867,12 @@ static __poll_t cgroup_pressure_poll(struct kernfs_open_file *of, > > return psi_trigger_poll(&ctx->psi.trigger, of->file, pt); > > } > > > > +static int cgroup_pressure_open(struct kernfs_open_file *of) > > +{ > > + return (of->file->f_mode & FMODE_WRITE && !capable(CAP_SYS_RESOURCE)) ? > > + -EPERM : 0; > > +} > > I agree with the change, but it's a bit unfortunate that this check is > duplicated between system and cgroup. > > What do you think about psi_trigger_create() taking the file and > checking FMODE_WRITE and CAP_SYS_RESOURCE against file->f_cred? That's definitely doable and we don't even need to pass file to psi_trigger_create() since it's called only when we write to the file. However by moving the capability check into psi_trigger_create() we also postpone the check until write() instead of failing early in open(). I always assumed failing early is preferable but if consolidating the code here makes more sense then I can make the switch. Please let me know if you still prefer to move the check.
