Re: [PATCH] vhost_task: do the return-to-userspace work between work items

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

On Sat, Oct 31, 2026 at 02:15:40PM -0400, Michael S. Tsirkin wrote:
> On Sat, Oct 31, 2026 at 11:10:04AM -0700, Shakeel Butt wrote:
> > A vhost task is a user worker: a thread of a user process that runs only
> > kernel code. It never returns to userspace, so it never runs the work
> > that is queued for the way back. resume_user_mode_work() does that work
> > when TIF_NOTIFY_RESUME is set. For a vhost task this means:
> > 
> > - memory.high is handled at the wrong time. When a charge goes over
> >   memory.high, the task counts the pages, and should reclaim, and sleep
> >   if needed, on its way back to userspace. A vhost task only reclaims
> >   when more than a batch of pages has built up and the charge can block.
> >   Then it reclaims and sleeps in the middle of the charge, maybe while
> >   it holds a lock. vhost-scsi, for example, can fault in guest memory
> >   while it holds vq->mutex. If the charges can't block, nothing is done
> >   at all, and the count keeps growing.
> > 
> > - Block cgroup throttling never runs. blkcg_schedule_throttle() asks for
> >   it, but only resume_user_mode_work() acts on it.
> > 
> > - Task work queued with TWA_RESUME, like a deferred fput() or a NUMA
> >   balancing scan, waits until the task gets a signal or exits.
> > 
> > io_uring workers had the same problem, and io_run_task_work() calls
> > resume_user_mode_work() for them. Do the same in vhost_task_fn(): if
> > TIF_NOTIFY_RESUME is set, call resume_user_mode_work() at the top of the
> > loop. That is between work items, no locks are held there, and the task
> > is running. Passing a NULL regs is fine, io_uring and KVM already do it.
> > 
> > This covers every user of vhost_task_create(): the vhost workers and
> > KVM's NX huge page recovery thread.
> > 
> > Tested in a VM with a version of tools/virtio/vhost_net_test that puts
> > each RX buffer on a new page, so the vhost worker faults in, and
> > charges, 256M while its cgroup has memory.high=64M. Before, all 285
> > calls to __mem_cgroup_handle_over_high() came from try_charge_memcg(),
> > inside handle_rx() with vq->mutex held. After, all 589 came from
> > vhost_task_fn(). memory.peak stayed at 64M, and the run took about the
> > same time (0.66s before, 0.67s after). A lockdep kernel showed no
> > warnings.
> > 
> > Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> 
> Is this different from:
> 
> Message-ID: <AS8P251MB00012469E6ECBEA2B7D87BB0C86C2@AS8P251MB0001.EURP251.PROD.OUTLOOK.COM>

Oh wow, it is same but somewhat for different reasons. I am fine that patch, I
will comment on it.