On Fri 12-02-21 11:42:15, David Hildenbrand wrote:

On 12.02.21 11:33, Michal Hocko wrote:

[...]

quoted

I have to digest this but my first impression is that this is more heavy
weight than it needs to. Pfn walkers should normally obey node range at
least. The first pfn is usually excluded but I haven't seen real

We've seen examples where this is not sufficient. Simple example:

Have your physical memory end within a memory section. Easy via QEMU, just
do a "-m 4000M". The remaining part of the last section has fake/wrong
node/zone info.

Does this really matter though. If those pages are reserved then nobody
will touch them regardless of their node/zone ids.

Hotplug memory. The node/zone gets resized such that PFN walkers might
stumble over it.

The basic idea is to make sure that any initialized/"online" pfn belongs to
exactly one node/zone and that the node/zone spans that PFN.

Yeah, this sounds like a good idea but what is the poper node for hole
between two ranges associated with a different nodes/zones? This will
always be a random number. We should have a clear way to tell "do not
touch those pages" and PageReserved sounds like a good way to tell that.

quoted

problems with that. The VM_BUG_ON blowing up is really bad but as said
above we can simply make it less offensive in presence of reserved pages
as those shouldn't reach that path AFAICS normally.

Andrea tried tried working around if via PG_reserved pages and it resulted
in quite some ugly code. Andrea also noted that we cannot rely on any random
page walker to do the right think when it comes to messed up node/zone info.

I am sorry, I haven't followed previous discussions. Has the removal of
the VM_BUG_ON been considered as an immediate workaround?
-- 
Michal Hocko
SUSE Labs