Re: [PATCH v2 1/3] mm/memory-failure: Add memory_failure_queue_kick()

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 18 May 2020 14:45:05 +0200 "Rafael J. Wysocki" <rjw@rjwysocki.net> wrote:

> On Friday, May 1, 2020 6:45:41 PM CEST James Morse wrote:
> > The GHES code calls memory_failure_queue() from IRQ context to schedule
> > work on the current CPU so that memory_failure() can sleep.
> > 
> > For synchronous memory errors the arch code needs to know any signals
> > that memory_failure() will trigger are pending before it returns to
> > user-space, possibly when exiting from the IRQ.
> > 
> > Add a helper to kick the memory failure queue, to ensure the scheduled
> > work has happened. This has to be called from process context, so may
> > have been migrated from the original cpu. Pass the cpu the work was
> > queued on.
> > 
> > Change memory_failure_work_func() to permit being called on the 'wrong'
> > cpu.
> > 
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -3012,6 +3012,7 @@ enum mf_flags {
> >  };
> >  extern int memory_failure(unsigned long pfn, int flags);
> >  extern void memory_failure_queue(unsigned long pfn, int flags);
> > +extern void memory_failure_queue_kick(int cpu);
> >  extern int unpoison_memory(unsigned long pfn);
> >  extern int get_hwpoison_page(struct page *page);
> >  #define put_hwpoison_page(page)	put_page(page)
> > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> > index a96364be8ab4..c4afb407bf0f 100644
> > --- a/mm/memory-failure.c
> > +++ b/mm/memory-failure.c
> > @@ -1493,7 +1493,7 @@ static void memory_failure_work_func(struct work_struct *work)
> >  	unsigned long proc_flags;
> >  	int gotten;
> >  
> > -	mf_cpu = this_cpu_ptr(&memory_failure_cpu);
> > +	mf_cpu = container_of(work, struct memory_failure_cpu, work);
> >  	for (;;) {
> >  		spin_lock_irqsave(&mf_cpu->lock, proc_flags);
> >  		gotten = kfifo_get(&mf_cpu->fifo, &entry);
> > @@ -1507,6 +1507,19 @@ static void memory_failure_work_func(struct work_struct *work)
> >  	}
> >  }
> >  
> > +/*
> > + * Process memory_failure work queued on the specified CPU.
> > + * Used to avoid return-to-userspace racing with the memory_failure workqueue.
> > + */
> > +void memory_failure_queue_kick(int cpu)
> > +{
> > +	struct memory_failure_cpu *mf_cpu;
> > +
> > +	mf_cpu = &per_cpu(memory_failure_cpu, cpu);
> > +	cancel_work_sync(&mf_cpu->work);
> > +	memory_failure_work_func(&mf_cpu->work);
> > +}
> > +
> >  static int __init memory_failure_init(void)
> >  {
> >  	struct memory_failure_cpu *mf_cpu;
> > 
> 
> I could apply this provided an ACK from the mm people.
> 

Naoya Horiguchi is the memory-failure.c person.  A review would be
appreciated please?

I'm struggling with it a bit.  memory_failure_queue_kick() should be
called on the cpu which is identified by arg `cpu', yes? 
memory_failure_work_func() appears to assume this.

If that's right then a) why bother passing in the `cpu' arg?  and b)
what keeps this thread pinned to that CPU?  cancel_work_sync() can
schedule.