Re: [PATCH for-7.5 5/5] memcg: simplify high limit handling after a charge

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Oct 14, 2026 at 01:46:23PM +0200, Michal Hocko wrote:
> On Sat 10-10-26 14:09:57, Shakeel Butt wrote:
> [...]
> > -	/* Interrupts, kernel threads, and remote chargers stop here. */
> > -	if (use_worker)
> > +	/*
> > +	 * Use a worker for interrupts, kernel threads, and tasks whose mm
> > +	 * does not match the charged hierarchy.
> > +	 */
> > +	if (use_high_work) {
> > +		if (!iter)
> > +			return;
> > +		if (allow_spinning)
> > +			schedule_work(&iter->high_work);
> > +		else
> > +			irq_work_queue(&iter->high_irq_work);
> >  		return;
> > +	}
> > +
> > +	if (iter) {
> 
> Would it be much simpler to bail out on iter==NULL (i.e. nothing over
> high limit) right after the loop?
> 

I want this patch to not change any behavior. If we return here when iter is
NULL, we will skip a potential call to __mem_cgroup_handle_over_high() when
current->memcg_nr_pages_over_high > MEMCG_CHARGE_BATCH. Though I agree that is
very subtle and most probably unexpected. If we decide to remove it, we should
do it in a separate patch. Actually I am working on a followup series where I
plan to remove the sync enforcement of memory.high and put high enforcement on
the code paths which can potentially do a lot of allocation within a single
kernel entry.

> > +		/*
> > +		 * Count the full batch so reclaim work and delay penalties scale
> > +		 * with the pages charged by this task.
> > +		 */
> > +		current->memcg_nr_pages_over_high += nr_pages;
> > +		set_notify_resume(current);
> > +	}
> >  
> >  	/*
> > -	 * Reclaim is set up above to be called from the userland
> > -	 * return path. But also attempt synchronous reclaim to avoid
> > -	 * excessive overrun while the task is still inside the
> > -	 * kernel. If this is successful, the return path will see it
> > -	 * when it rechecks the overage and simply bail out.
> > +	 * Earlier charges may have left high overage pending even if this charge
> > +	 * found no new overage. Handle it now if this charge can block, to limit
> > +	 * overrun while the task remains in the kernel.
> >  	 */
> >  	if (current->memcg_nr_pages_over_high > MEMCG_CHARGE_BATCH &&
> >  	    !(current->flags & PF_MEMALLOC) &&
> > -- 
> > 2.53.0-Meta
> 
> -- 
> Michal Hocko
> SUSE Labs