Re: [PATCH v4 08/14] mm/gup: grab head page refcount once for group of subpages

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On Thu, Oct 14, 2021 at 06:56:51PM +0100, Joao Martins wrote:

> > And I would document this relationship in the GUP side "This do/while
> > is required because insert_pfn_pmd/pud() is used with compound pages
> > smaller than the PUD/PMD size" so it isn't so confused with just
> > "devmap"
> 
> Also, it's not that PMDs span compound heads, it's that PMDs/PUDs use
> just base pages. Compound pages/head in devmap are only introduced by
> series and for device-dax only.

I think it all makes sense, I just want to clarify what I mean by
compound_head:

  compound_head(base_page) == base_page

Ie a PMD consisting only of order 0 pages would have N 'compound_heads'
within it even though nothing is a compound page.

Which is relavent because the GUP algorithms act on the compound_head

Jason




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