Re: collision between ZONE_MOVABLE and memblock allocations

[Mike Rapoport]

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.

-- 
Sincerely yours,
Mike.