Re: [PATCH] migration/mm: Add WARN_ON to try_offline_node

[Michal Hocko]

On Tue 02-10-18 12:45:50, Tyrel Datwyler wrote:
> On 10/02/2018 11:13 AM, Michael Bringmann wrote:
> > 
> > 
> > On 10/02/2018 11:04 AM, Michal Hocko wrote:
> >> On Tue 02-10-18 10:14:49, Michael Bringmann wrote:
> >>> On 10/02/2018 09:59 AM, Michal Hocko wrote:
> >>>> On Tue 02-10-18 09:51:40, Michael Bringmann wrote:
> >>>> [...]
> >>>>> When the device-tree affinity attributes have changed for memory,
> >>>>> the 'nid' affinity calculated points to a different node for the
> >>>>> memory block than the one used to install it, previously on the
> >>>>> source system.  The newly calculated 'nid' affinity may not yet
> >>>>> be initialized on the target system.  The current memory tracking
> >>>>> mechanisms do not record the node to which a memory block was
> >>>>> associated when it was added.  Nathan is looking at adding this
> >>>>> feature to the new implementation of LMBs, but it is not there
> >>>>> yet, and won't be present in earlier kernels without backporting a
> >>>>> significant number of changes.
> >>>>
> >>>> Then the patch you have proposed here just papers over a real issue, no?
> >>>> IIUC then you simply do not remove the memory if you lose the race.
> >>>
> >>> The problem occurs when removing memory after an affinity change
> >>> references a node that was previously unreferenced.  Other code
> >>> in 'kernel/mm/memory_hotplug.c' deals with initializing an empty
> >>> node when adding memory to a system.  The 'removing memory' case is
> >>> specific to systems that perform LPM and allow device-tree changes.
> >>> The powerpc kernel does not have the option of accepting some PRRN
> >>> requests and accepting others.  It must perform them all.
> >>
> >> I am sorry, but you are still too cryptic for me. Either there is a
> >> correctness issue and the the patch doesn't really fix anything or the
> >> final race doesn't make any difference and then the ppc code should be
> >> explicit about that. Checking the node inside the hotplug core code just
> >> looks as a wrong layer to mitigate an arch specific problem. I am not
> >> saying the patch is a no-go but if anything we want a big fat comment
> >> explaining how this is possible because right now it just points to an
> >> incorrect API usage.
> >>
> >> That being said, this sounds pretty much ppc specific problem and I
> >> would _prefer_ it to be handled there (along with a big fat comment of
> >> course).
> > 
> > Let me try again.  Regardless of the path to which we get to this condition,
> > we currently crash the kernel.  This patch changes that to a WARN_ON notice
> > and continues executing the kernel without shutting down the system.  I saw
> > the problem during powerpc testing, because that is the focus of my work.
> > There are other paths to this function besides powerpc.  I feel that the
> > kernel should keep running instead of halting.
> 
> This is still basically a hack to get around a known race. In itself
> this patch is still worth while in that we shouldn't crash the kernel
> on a null pointer dereference. However, I think the actual problem
> still needs to be addressed. We shouldn't run any PRRN events for the
> source system on the target after a migration. The device tree update
> should have taken care of telling us about new affinities and what
> not. Can we just throw out any queued PRRN events when we wake up on
> the target?

And until a proper fix is developed can we have NODE_DATA test in the
affected code rather than pollute the generic code with something that
is essentially a wrong usage of the API? With a big fat warning
explaining what is going on here?
-- 
Michal Hocko
SUSE Labs