Re: [PATCH 2/2] mm,memory_hotplug: Fix shrink_{zone,node}_span

[Oscar Salvador]

On Wed, Jul 17, 2019 at 10:01:01AM +0200, David Hildenbrand wrote:
> I'd also like to note that we should strive for making all zone-related
> changes when offlining in the future, not when removing memory. So
> ideally, any core changes we perform from now, should make that step
> (IOW implementing that) easier, not harder. Of course, BUGs have to be
> fixed.
> 
> The rough idea would be to also mark ZONE_DEVICE sections as ONLINE (or
> rather rename it to "ACTIVE" to generalize).
> 
> For each section we would then have
> 
> - pfn_valid(): We have a valid "struct page" / memmap
> - pfn_present(): We have actually added that memory via an oficial
>   interface to mm (e.g., arch_add_memory() )
> - pfn_online() / (or pfn_active()): Memory is active (online in "buddy"-
>   speak, or memory that was moved to the ZONE_DEVICE zone)
> 
> When resizing the zones (e.g., when offlining memory), we would then
> search for pfn_online(), not pfn_present().
> 
> In addition to move_pfn_range_to_zone(), we would have
> remove_pfn_range_from_zone(), called during offline_pages() / by
> devmem/hmm/pmem code before removing.
> 
> (I started to look into this, but I don't have any patches yet)

Yes, it seems like a good approach, and something that makes sense
to pursue.
FWIF, I sent a patchset [1] for that a long time ago, but I could not
follow-up due to time constraints.

[1] https://patchwork.kernel.org/cover/10700783/


-- 
Oscar Salvador
SUSE L3