Re: [PATCH] kho: add support for deferred struct page init

[Pratyush Yadav <pratyush@kernel.org>]

On Tue, Dec 23 2025, Pasha Tatashin wrote:

>> > if (WARN_ON_ONCE(info.magic != KHO_PAGE_MAGIC || info.order > MAX_PAGE_ORDER))
>> > return NULL;
>>
>> See my patch that drops this restriction:
>> https://lore.kernel.org/linux-mm/20251206230222.853493-2-pratyush@kernel.org/
>>
>> I think it was wrong to add it in the first place.
>
> Agree, the restriction can be removed. Indeed, it is wrong as it is
> not enforced during preservation.
>
> However, I think we are going to be in a world of pain if we allow
> preserving memory from different topologies within the same order. In
> kho_preserve_pages(), we have to check if the first and last page are
> from the same nid; if not, reduce the order by 1 and repeat until they
> are. It is just wrong to intermix different memory into the same
> order, so in addition to removing that restriction, I think we should
> implement this enforcement.

Sure, makes sense.

>
> Also, perhaps we should pass the NID in the Jason's radix tree
> together with the order. We could have a single tree that encodes both
> order and NID information in the top level, or we can have one tree
> per NID. It does not really matter to me, but that should help us with
> faster struct page initialization.

Can we use NIDs in ABI? Do they stay stable across reboots? I never
looked at how NIDs actually get assigned.

Not sure if we should target it for the initial merge of the radix tree,
but I think this is something we can try to figure out later down the
line.

>
>> >> To get the nid, you would need to call early_pfn_to_nid(). This takes a
>> >> spinlock and searches through all memblock memory regions. I don't think
>> >> it is too expensive, but it isn't free either. And all this would be
>> >> done serially. With the zone search, you at least have some room for
>> >> concurrency.
>> >>
>> >> I think either approach only makes a difference when we have a large
>> >> number of low-order preservations. If we have a handful of high-order
>> >> preservations, I suppose the overhead of nid search would be negligible.
>> >
>> > We should be targeting a situation where the vast majority of the
>> > preserved memory is HugeTLB, but I am still worried about lower order
>> > preservation efficiency for IOMMU page tables, etc.
>>
>> Yep. Plus we might get VMMs stashing some of their state in a memfd too.
>
> Yes, that is true, but hopefully those are tiny compared to everything else.
>
>> >> Long term, I think we should hook this into page_alloc_init_late() so
>> >> that all the KHO pages also get initalized along with all the other
>> >> pages. This will result in better integration of KHO with rest of MM
>> >> init, and also have more consistent page restore performance.
>> >
>> > But we keep KHO as reserved memory, and hooking it up into
>> > page_alloc_init_late() would make it very different, since that memory
>> > is part of the buddy allocator memory...
>>
>> The idea I have is to have a separate call in page_alloc_init_late()
>> that initalizes KHO pages. It would traverse the radix tree (probably in
>> parallel by distributing the address space across multiple threads?) and
>> initialize all the pages. Then kho_restore_page() would only have to
>> double-check the magic and it can directly return the page.
>
> I kind of do not like relying on magic to decide whether to initialize
> the struct page. I would prefer to avoid this magic marker altogether:
> i.e. struct page is either initialized or not, not halfway
> initialized, etc.

The magic is purely sanity checking. It is not used to decide anything
other than to make sure this is actually a KHO page. I don't intend to
change that. My point is, if we make sure the KHO pages are properly
initialized during MM init, then restoring can actually be a very cheap
operation, where you only do the sanity checking. You can even put the
magic check behind CONFIG_KEXEC_HANDOVER_DEBUG if you want, but I think
it is useful enough to keep in production systems too.

>
> Magic is not reliable. During machine reset in many firmware
> implementations, and in every kexec reboot, memory is not zeroed. The
> kernel usually allocates vmemmap using exactly the same pages, so
> there is just too high a chance of getting magic values accidentally
> inherited from the previous boot.

I don't think that can happen. All the pages are zeroed when
initialized, which will clear the magic. We should only be setting the
magic on an initialized struct page.

>
>> Radix tree makes parallelism easier than the linked lists we have now.
>
> Agree, radix tree can absolutely help with parallelism.
>
>> >> Jason's radix tree patches will make that a bit easier to do I think.
>> >> The zone search will scale better I reckon.
>> >
>> > It could, perhaps early in boot we should reserve the radix tree, and
>> > use it as a source of truth look-ups later in boot?
>>
>> Yep. I think the radix tree should mark its own pages as preserved too
>> so they stick around later in boot.
>
> Unfortunately, this can only be done in the new kernel, not in the old
> kernel; otherwise we can end up with a recursive dependency that may
> never be satisfied.

Right. It shouldn't be too hard to do in the new kernel though. We will
walk the whole tree anyway.

-- 
Regards,
Pratyush Yadav