Re: PageOffline: refcount, flags and memdesc

[David Hildenbrand <david@redhat.com>]

>>
>> Instead of alloc_frozen_pages(), I was wondering if we should have something
>> like GFP_FROZEN. For example, for two PG_offline users
>> I'd currently also need alloc_contig_frozen_range() and
>> alloc_contig_frozen_pages(). Using alloc_contig_range(GFP_FROZEN)
>> alloc_contig_pages(GFP_PROZEN) would make that easier.
>>
>> Did you consider that already?
> 
> I think Yu Zhao's recent patches:
> 
> e98337d11bbd ("mm/contig_alloc: support __GFP_COMP")
> 463586e9ff39 ("mm/cma: add cma_{alloc,free}_folio()")
> cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP for gigantic folios")
> 
> get us most of the way there.
> 
> What I really want to do is make contig allocation not do anything with
> the refcounts.  Hoist that into the callers which actually want it
> (probably not many), and then we can actually drop the __GFP_COMP
> support to alloc_contig_range_noprof().

Right, at least virtio-mem cannot easily support 
alloc_contig_pages(__GFP_COMP; we'd need splitting support, and it 
should be a separate effort than reworking PageOffline; and it would all 
be in vain if we drop __GFP_COMP later again :

So regarding the PageOffline refcount rework it would be easier to do it 
stepwise

1) add GFP_FROZEN support to alloc_contig_*
2) convert the two alloc_contig_*+PageOffline users to use that
3) convert the remaining alloc_contig_* users to GFP_FROZEN
4) make GFP_FROZEN implicit (drop it?) and drop __GFP_COMP

3) is a bigger task, likely too big to do in one shot.

-- 
Cheers,

David / dhildenb