Re: collision between ZONE_MOVABLE and memblock allocations

[Michal Hocko]

On Wed 26-07-23 16:23:17, Mike Rapoport wrote:
> On Wed, Jul 26, 2023 at 02:57:55PM +0200, Michal Hocko wrote:
> > On Wed 26-07-23 13:48:45, Mike Rapoport wrote:
> > > On Wed, Jul 26, 2023 at 09:49:12AM +0200, Michal Hocko wrote:
> > > > On Fri 21-07-23 14:20:09, Mike Rapoport wrote:
> > > > > On Wed, Jul 19, 2023 at 04:26:04PM -0600, Ross Zwisler wrote:
> > > > > > On Wed, Jul 19, 2023 at 08:44:34AM +0300, Mike Rapoport wrote:
> > > > > > > 3. Switch memblock to use bottom up allocations. Historically memblock
> > > > > > > allocated memory from the top to avoid corrupting the kernel image and to
> > > > > > > avoid exhausting precious ZONE_DMA. I believe we can use bottom-up
> > > > > > > allocations with lower limit of memblock allocations set to 16M.
> > > > > > > 
> > > > > > > With the hack below no memblock allocations will end up in ZONE_MOVABLE:
> > > > > > 
> > > > > > Yep, I've confirmed that for my use cases at least this does the trick, thank
> > > > > > you!  I had thought about moving the memblock allocations, but had no idea it
> > > > > > was (basically) already supported and thought it'd be much riskier than just
> > > > > > adjusting where ZONE_MOVABLE lived.
> > > > > > 
> > > > > > Is there a reason for this to not be a real option for users, maybe per a
> > > > > > kernel config knob or something?  I'm happy to explore other options in this
> > > > > > thread, but this is doing the trick so far.
> > > > > 
> > > > > I think we can make x86 always use bottom up.
> > > > > 
> > > > > To do this properly we'd need to set lower limit for memblock allocations
> > > > > to MAX_DMA32_PFN and allow fallback below it so that early allocations
> > > > > won't eat memory from ZONE_DMA32.
> > > > > 
> > > > > Aside from x86 boot being fragile in general I don't see why this wouldn't
> > > > > work.
> > > > 
> > > > This would add a very subtle depency of a functionality on the specific
> > > > boot allocator behavior and that is bad for long term maintenance.
> > > 
> > > What do you mean by "specific boot allocator behavior"?
> > 
> > I mean that the expectation that the boot allocator starts from low
> > addresses and functionality depending on that is too fragile. This has
> > already caused some problems in the past IIRC.
> 
> Well, any change in x86 boot sequence may cause all sorts of problems :)
> 
> We do some of the boot time allocations from low addresses when
> movable_node is enabled and that is entirely implicit and buried deep
> inside the code.
> 
> What I'm suggesting is to switch the allocations to bottom-up once and for
> all with explicitly set lower limit and a defined semantics for a fallback.
>  
> This might cause some bumps in the beginning, but I don't expect it to be a
> maintenance problem in the long run.
> 
> And it will free higher memory from early allocations for all usecases, not
> just this one.

Higher memory is usually not a problem AFAIK. It is lowmem that is a
more scarce resource because some HW might be constrained in why phys
address range is visible.

-- 
Michal Hocko
SUSE Labs