Re: [PATCH] mm: memory_hotplug: put migration failure information under DEBUG_VM

[Vlastimil Babka]

On 11/23/20 4:10 PM, Charan Teja Kalla wrote:
> Thanks Michal!
> On 11/23/2020 7:43 PM, Michal Hocko wrote:
> > On Mon 23-11-20 19:33:16, Charan Teja Reddy wrote:
> > > When the pages are failed to get isolate or migrate, the page owner
> > > information along with page info is dumped. If there are continuous
> > > failures in migration(say page is pinned) or isolation, the log buffer
> > > is simply getting flooded with the page owner information. As most of
> > > the times page info is sufficient to know the causes for failures of
> > > migration or isolation, place the page owner information under DEBUG_VM.
> >
> > I do not see why this path is any different from others that call
> > dump_page. Page owner can add a very valuable information to debug
> > the underlying reasons for failures here. It is an opt-in debugging
> > feature which needs to be enabled explicitly. So I would argue users
> > are ready to accept a lot of data in the kernel log.
>
> Just thinking how frequently failures can happen in those paths. In the
> memory hotplug path, we can flood the page owner logs just by making one
> page pinned. Say If it is anonymous page, the page owner information

So you say it's flooded when page_owner info is included, but not 
flooded when only the rest of __dump_page() is printed? (which is also 
not just one or two lines). That has to be very specific rate of failures.
Anyway I don't like the solution with arbitrary config option. To 
prevent flooding we generally have ratelimiting, how about that?

Also agreed with Michal that page_owner is explicitly enabled debugging 
option and if you use it in production, that's rather surprising to me, 
and possibly more rare than DEBUG_VM, which IIRC Fedora kernels use.