Re: [PATCH v7 2/3] kho: fix deferred init of kho scratch

[Zi Yan <ziy@nvidia.com>]

On 18 Mar 2026, at 13:19, Michał Cłapiński wrote:

> On Wed, Mar 18, 2026 at 6:08 PM Zi Yan <ziy@nvidia.com> wrote:
>>
>> On 18 Mar 2026, at 11:45, Michał Cłapiński wrote:
>>
>>> On Wed, Mar 18, 2026 at 4:26 PM Zi Yan <ziy@nvidia.com> wrote:
>>>>
>>>> On 18 Mar 2026, at 11:18, Michał Cłapiński wrote:
>>>>
>>>>> On Wed, Mar 18, 2026 at 4:10 PM Zi Yan <ziy@nvidia.com> wrote:
>>>>>>
>>>>>> On 17 Mar 2026, at 10:15, Michal Clapinski wrote:
>>>>>>
>>>>>>> Currently, if DEFERRED is enabled, kho_release_scratch will initialize
>>>>>>> the struct pages and set migratetype of kho scratch. Unless the whole
>>>>>>> scratch fit below first_deferred_pfn, some of that will be overwritten
>>>>>>> either by deferred_init_pages or memmap_init_reserved_pages.
>>>>>>>
>>>>>>> To fix it, I modified kho_release_scratch to only set the migratetype
>>>>>>> on already initialized pages. Then, modified init_pageblock_migratetype
>>>>>>> to set the migratetype to CMA if the page is located inside scratch.
>>>>>>>
>>>>>>> Signed-off-by: Michal Clapinski <mclapinski@google.com>
>>>>>>> ---
>>>>>>>  include/linux/memblock.h           |  2 --
>>>>>>>  kernel/liveupdate/kexec_handover.c | 10 ++++++----
>>>>>>>  mm/memblock.c                      | 22 ----------------------
>>>>>>>  mm/page_alloc.c                    |  7 +++++++
>>>>>>>  4 files changed, 13 insertions(+), 28 deletions(-)
>>>>>>>
>>>>>>
>>>>>> <snip>
>>>>>>
>>>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>>>>> index ee81f5c67c18..5ca078dde61d 100644
>>>>>>> --- a/mm/page_alloc.c
>>>>>>> +++ b/mm/page_alloc.c
>>>>>>> @@ -55,6 +55,7 @@
>>>>>>>  #include <linux/cacheinfo.h>
>>>>>>>  #include <linux/pgalloc_tag.h>
>>>>>>>  #include <linux/mmzone_lock.h>
>>>>>>> +#include <linux/kexec_handover.h>
>>>>>>>  #include <asm/div64.h>
>>>>>>>  #include "internal.h"
>>>>>>>  #include "shuffle.h"
>>>>>>> @@ -549,6 +550,12 @@ void __meminit init_pageblock_migratetype(struct page *page,
>>>>>>>                    migratetype < MIGRATE_PCPTYPES))
>>>>>>>               migratetype = MIGRATE_UNMOVABLE;
>>>>>>>
>>>>>>> +     /*
>>>>>>> +      * Mark KHO scratch as CMA so no unmovable allocations are made there.
>>>>>>> +      */
>>>>>>> +     if (unlikely(kho_scratch_overlap(page_to_phys(page), PAGE_SIZE)))
>>>>>>> +             migratetype = MIGRATE_CMA;
>>>>>>> +
>>>>>>
>>>>>> If this is only for deferred init code, why not put it in deferred_free_pages()?
>>>>>> Otherwise, all init_pageblock_migratetype() callers need to pay the penalty
>>>>>> of traversing kho_scratch array.
>>>>>
>>>>> Because reserve_bootmem_region() doesn't call deferred_free_pages().
>>>>> So I would also have to modify it.
>>>>>
>>>>> And the early initialization won't pay the penalty of traversing the
>>>>> kho_scratch array, since then kho_scratch is NULL.
>>>>
>>>> How about hugetlb_bootmem_init_migratetype(), init_cma_pageblock(),
>>>> init_cma_reserved_pageblock(), __init_page_from_nid(), memmap_init_range(),
>>>> __init_zone_device_page()?
>>>>
>>>> 1. are they having any PFN range overlapping with kho?
>>>> 2. is kho_scratch NULL for them?
>>>>
>>>> 1 tells us whether putting code in init_pageblock_migratetype() could save
>>>> the hassle of changing all above locations.
>>>> 2 tells us how many callers are affected by traversing kho_scratch.
>>>
>>> I could try answering those questions but
>>>
>>> 1. I'm new to this and I'm not sure how correct the answers will be.
>>>
>>> 2. If you're not using CONFIG_KEXEC_HANDOVER, the performance penalty
>>> will be zero.
>>> If you are using it, currently you have to disable
>>> CONFIG_DEFERRED_STRUCT_PAGE_INIT and the performance hit from this is
>>> far, far greater. This solution saves 0.5s on my setup (100GB of
>>> memory). We can always improve the performance further in the future.
>>>
>>
>> OK, I asked Claude for help and the answer is that not all callers of
>> init_pageblock_migratetype() touch kho scratch memory regions. Basically,
>> you only need to perform the kho_scratch_overlap() check in
>> __init_page_from_nid() to achieve the same end result.
>>
>>
>> The below is the analysis from Claude.
>> Based on my understanding,
>> 1. memmap_init_range() is done before kho_memory_init(), so it does not need
>> the check.
>>
>> 2. __init_zone_device_page() is not relevant.
>>
>> 3. init_cma_reserved_pageblock() / init_cma_pageblock() are already set
>> to MIGRATE_CMA.
>>
>> 4. hugetlb is not used by kho scratch, so also does not need the check.
>>
>> 5. kho_release_scratch() already takes care of it.
>>
>> The remaining memblock_free_pages() needs a check, but I am not 100%.
>>
>>
>> # kho_scratch_overlap() in init_pageblock_migratetype() — scope analysis
>>
>> ## Context
>>
>> Commit a7700b3c6779 ("kho: fix deferred init of kho scratch") added a
>> kho_scratch_overlap() call inside init_pageblock_migratetype() in
>> mm/page_alloc.c:
>>
>> ```c
>> if (unlikely(kho_scratch_overlap(page_to_phys(page), PAGE_SIZE)))
>>     migratetype = MIGRATE_CMA;
>> ```
>>
>> kho_scratch_overlap() does a NULL check followed by a loop over the
>> kho_scratch array. For non-KHO boots (kho_scratch == NULL) the cost is
>> a single NULL load and branch. For KHO boots the loop runs on every call
>> to init_pageblock_migratetype().
>>
>> ## Question
>>
>> Does this add overhead for callers whose memory range cannot overlap
>> with scratch? Can the check be moved to the caller side?
>>
>> ## Call site analysis
>>
>> init_pageblock_migratetype() has nine call sites. The init call ordering
>> relevant to scratch is:
>>
>> ```
>> setup_arch()
>>   zone_sizes_init() -> free_area_init() -> memmap_init_range()   [1]
>>
>> mm_init_free_all() / start_kernel():
>>   kho_memory_init() -> kho_release_scratch()                     [2]
>>   memblock_free_all()
>>     free_low_memory_core_early()
>>       memmap_init_reserved_pages()
>>         reserve_bootmem_region() -> __init_deferred_page()
>>           -> __init_page_from_nid()                              [3]
>>   deferred init kthreads -> __init_page_from_nid()               [4]
>> ```
>
> I don't understand this. deferred_free_pages() doesn't call
> __init_page_from_nid(). So I would clearly need to modify both
> deferred_free_pages and __init_page_from_nid.

Sure. But other callers I mentioned above do not need to check kho_scratch,
right?


Best Regards,
Yan, Zi