Re: [PATCH] kho: Prevent reuse of preserved pages after a reservation failure

[Mike Rapoport]

On Wed, Nov 11, 2026 at 12:16:43AM -0800, Chris Down wrote:
> Hi Mike,
> 
> Mike Rapoport writes:
> > On Tue, Nov 10, 2026 at 11:09:30PM -0800, Chris Down wrote:
> > > kho_preserved_memory_reserve() reserves each range of preserved memory
> > > in memblock and marks it noinit, but ignores both return values. Either
> > > can fail when memblock has to grow its reserved array and can't find
> > > room, which at this point in boot it may only take from KHO bootmem.
> > 
> > Is it an LLM analysis or you seen this happen?
> 
> Are those the only two options nowadays! :-) In this case it's just eyeballs.

Didn't think anybody still uses those :)
Kudos!
 
> I happened to notice this while preparing my KHO/LUO observability patchset
> and thought it made sense to send as a separate patch, since it doesn't
> depend on anything there.
> 
> It's probably realistically quite unlikely, I would think it would require a
> lot of fragmentation or a very small kho_bootmem=.

It's unlikely indeed and if that happens we'll panic on memory map
allocation anyway :)
 
> > > Dropping the handover at this point isn't safe either. reserve_mem=
> > > revives its regions from the handover before mm_core_init(), and
> > > relies on this walk to reserve them.
> > 
> > reserve_mem_kho_revive() can memblock_reserve() its memory itself and we
> > can carry on, can't we?
> 
> Thanks, that's better. I had thought carryong on wasn't possible because
> reserve_mem has already reserved its regions by the time we get to the walk,
> but actually you're right that there's no reason reserve_mem_kho_revive()
> can't just do it itself...
> 
> Looking at the code one thing we would likely trip over then is that
> kho_mem_retrieve()'s error path only clears fdt_phys, but not bootmem_phys.
> kho_init() then thinks that it's a cold boot and runs
> init_cma_reserved_pageblock() over the inherited bootmem which has already
> been handed to the buddy allocator, so this would need adjustment first.
> 
> So then, let me send v2 with reserve_mem_kho_revive() sorting out its own
> range, and making kho_init() skip the bootmem setup when the bootmem was
> inherited, if that sounds good?

Yeah, just keep them as two patches please :)
 
> Thanks!
> 
> Chris

-- 
Sincerely yours,
Mike.