Re: [PATCH v2 0/5] kdump: crashkernel reservation from CMA

[Michal Hocko <mhocko@suse.com>]

On Fri 30-05-25 10:06:52, David Hildenbrand wrote:
> On 29.05.25 09:46, Michal Hocko wrote:
> > On Wed 28-05-25 23:01:04, David Hildenbrand wrote:
> > [...]
> > > I think we just have to be careful to document it properly -- especially the
> > > shortcomings and that this feature might become a problem in the future.
> > > Movable user-space page tables getting placed on CMA memory would probably
> > > not be a problem if we don't care about ... user-space data either way.
> > 
> > I think makedumpfile could refuse to capture a dump if userspace memory
> > is requested to enforce this.
> 
> Yeah, it will be tricky once we support placing other memory on CMA regions.
> E.g., there was the discussion of making some slab allocations movable.
> 
> But probably, in such a configuration, we would later simply refuse to
> active CMA kdump.

Or we can make the kdump CMA region more special and only allow
GFP_HIGHUSER_MOVABLE allocations from that. Anyaway I think we should
deal with this once we get there.
 
> > > The whole "Direct I/O takes max 1s" part is a bit shaky. Maybe it could be
> > > configurable how long to wait? 10s is certainly "safer".
> > 
> > Quite honestly we will never know and rather than making this
> > configurable I would go with reasonably large. Couple of seconds
> > certainly do not matter for the kdump situations but I would go as far
> > as minutes.
> 
> I recall that somebody raised that kdump downtime might be problematic
> (might affect service downtime?).
> 
> So I would just add a kconfig option with a default of 10s.

kconfig option usually doesn't really work for distro kernels. I am
personally not really keen on having a tuning knob because there is a
risk of cargo cult based tuning we have seen in other areas. That might
make it hard to remove the knob later on. Fundamentally we should have 2
situations though. Either we know that the HW is sane and then we
shouldn't really need any sleep or the HW might misbehave and then we
need to wait _some_ time. If our initial guess is incorrect then we can
always increase it and we would learn about that through bug reports.

All that being said I would go with an additional parameter to the
kdump cma setup - e.g. cma_sane_dma that would skip waiting and use 10s
otherwise. That would make the optimized behavior opt in, we do not need
to support all sorts of timeouts and also learn if this is not
sufficient.

Makes sense?
-- 
Michal Hocko
SUSE Labs