Re: [PATCHv2 02/14] mm/sparse: Check memmap alignment

["David Hildenbrand (Red Hat)" <david@kernel.org>]

>> For quite some time there will be a magical config option that will switch
>> between both layouts. I'd assume that things will get more complicated if we
>> suddenly have a "compound_head/folio" pointer and a "compound_info" pointer
>> at the same time.
>>
>> But it's really Willy who has the concept in mind as he is very likely right
>> now busy writing some of that code.
>>
>> I'm just the messenger.
>>
>> :)
>>
>> [I would hope that Willy could share his thoughts]
> 
> If you or Willy think that this patch will impede memdesc progress, I am
> okay not pushing this patchset upstream.

I pinged Willy.

> 
> I was really excited when I found this trick to get rid of fake heads.
> But ultimately, it is a clean up. I failed to find a performance win I
> hoped for.

I think it's quite nice as a cleanup, and if we wouldn't have memdescs 
on the horizon that essentially change the code completely in another 
direction (having all pages point to a struct folio, not just the tail 
pages), I wouldn't be bringing this up :)

> 
> Also, I try to understand what 32-byte layout means for fake heads.
> _refcount in struct page is going to 0 and refcounting happens on folios.

Yes, for folios.

> So I wounder if we can all pages identical (no tail pages per se) and
> avoid fake heads this way?

That's the ultimate goal, yes. Essentially, all pages will point to the
memdesc, and there will not be a reason to check for head/fake-head etc.

I think initially, the compound-page concept might
still co-exist for some memdescs that we won't initially allocate 
separately.
But I don't know the details of that.

I know that the transition phase is tricky :)

Regarding reference and folios: yes exactly. When trying to get a 
reference, we'll spot in the memdesc field that this is a folio and try 
on the folio instead.

In the future, most pages will either be permanently frozen and not have 
a refcount (e.g., struct ptdesc), or have a refcount in their memdesc. 
In the transition, the location of the refcount depends on memdesc type 
(in memdesc vs. in page).

-- 
Cheers

David