Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

[Qian Cai]

On Thu, 2019-10-10 at 19:30 +0200, Michal Hocko wrote:
> On Thu 10-10-19 10:47:38, Qian Cai wrote:
> > On Thu, 2019-10-10 at 16:18 +0200, Michal Hocko wrote:
> > > On Thu 10-10-19 09:11:52, Qian Cai wrote:
> > > > On Thu, 2019-10-10 at 12:59 +0200, Michal Hocko wrote:
> > > > > On Thu 10-10-19 05:01:44, Qian Cai wrote:
> > > > > > 
> > > > > > 
> > > > > > > On Oct 9, 2019, at 12:23 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > > > > > > 
> > > > > > > If this was only about the memory offline code then I would agree. But
> > > > > > > we are talking about any printk from the zone->lock context and that is
> > > > > > > a bigger deal. Besides that it is quite natural that the printk code
> > > > > > > should be more universal and allow to be also called from the MM
> > > > > > > contexts as much as possible. If there is any really strong reason this
> > > > > > > is not possible then it should be documented at least.
> > > > > > 
> > > > > > Where is the best place to document this? I am thinking about under
> > > > > > the âstruct zoneâ definitionâs lock field in mmzone.h.
> > > > > 
> > > > > I am not sure TBH and I do not think we have reached the state where
> > > > > this would be the only way forward.
> > > > 
> > > > How about I revised the changelog to focus on memory offline rather than making
> > > > a rule that nobody should call printk() with zone->lock held?
> > > 
> > > If you are to remove the CONFIG_DEBUG_VM printk then I am all for it. I
> > > am still not convinced that fiddling with dump_page in the isolation
> > > code is justified though.
> > 
> > No, dump_page() there has to be fixed together for memory offline to be useful.
> > What's the other options it has here?
> 
> I would really prefer to not repeat myself
> http://lkml.kernel.org/r/20191010074049.GD18412@xxxxxxxxxxxxxx

Care to elaborate what does that mean? I am confused on if you finally agree on
no printk() while held zone->lock or not. You said "If there is absolutely
no way around that then we might have to bite a bullet and consider some
of MM locks a land of no printk." which makes me think you agreed, but your
stance from the last reply seems you were opposite to it.

> 
> > By not holding zone->lock in dump_page()
> > from set_migratetype_isolate(), it even has a good side-effect to increase the
> > system throughput as dump_page() could be time-consuming. It may make the code a
> > bit cleaner by introducing a has_unmovable_pages_locked() version.
> 
> I do not see why we should really optimize this cold path.