Re: [PATCH v7 1/2] mm/memory hotplug: fix zone->contiguous always false when hotplug

[Li, Tianyou]

Very appreciated for your review David! The suggestions you made are 
clear and the code/comments you posted are well formatted, I can even 
copy/pasted without modification. Thanks.


Regards,

Tianyou

On 1/7/2026 4:03 AM, David Hildenbrand (Red Hat) wrote:
> On 12/22/25 15:58, Tianyou Li wrote:
> > Function set_zone_contiguous used __pageblock_pfn_to_page to
> > check the whole pageblock is in the same zone. One assumption is
> > the memory section must online, otherwise the __pageblock_pfn_to_page
> > will return NULL, then the set_zone_contiguous will be false.
> > When move_pfn_range_to_zone invoked set_zone_contiguous, since the
> > memory section did not online, the return value will always be false.
> >
> > To fix this issue, we removed the set_zone_contiguous from the
> > move_pfn_range_to_zone, and place it after memory section onlined.
> >
> > Function remove_pfn_range_from_zone did not have this issue because
> > memory section remains online at the time set_zone_contiguous invoked.
>
> The description is a bit hard to follow. Let me try:
>
>
> "set_zone_contiguous() uses __pageblock_pfn_to_page() to detect 
> pageblocks that either do not exist (hole) or that do not belong to 
> the same zone.
>
> __pageblock_pfn_to_page(), however, relies on pfn_to_online_page(), 
> effectively always returning NULL for memory ranges that were not 
> onlined yet. So when called on a range-to-be-onlined, it indicates a 
> memory hole to set_zone_contiguous().
>
> Consequently, the set_zone_contiguous() call in 
> move_pfn_range_to_zone(), which happens early during memory onlining, 
> will never detect a zone as being contiguous. Bad.
>
> To fix the issue, move the set_zone_contiguous() call to a later stage
> in memory onlining, where pfn_to_online_page() will succeed: after we
> mark the memory sections to be online"
Will change accordingly.  Thanks.


> Now, there is no need to add the handling to 
> mhp_init_memmap_on_memory(). Note how mhp_init_memmap_on_memory() in 
> memory_block_online() is always followed by online_pages().
Plus there is no dependencies of previous zone contiguous state for the 
set_zone_contiguous now, it totally makes sense to remove the 
set_zone_contiguous in mhp_init_memmap_on_memory() as you suggested.


> So, it's sufficient to move it after the online_pages_range(). I would 
> also add a comment there saying something like:
>
> /*
>  * Now that the ranges are indicated as online, check whether the whole
>  * zone is contiguous.
>  */
Will change accordingly. Thanks.


> Can we find some Fixes: tag (which commit introduced the regression)? 
> Likely we want to CC stable.
Yes, probably we can add the tags as below, where the pfn_to_page() 
changed to pfn_to_online_page() in __pageblock_pfn_to_page().

Fixes: 2d070eab2e82 (mm: consider zone which is not fully populated to 
have holes)
Cc: Michal Hocko <mhocko@xxxxxxxx>