Re: [PATCHv5 04/28] mm, thp: adjust conditions when we can reuse the page on WP fault

[Hillf Danton]

> 
> With new refcounting we will be able map the same compound page with
> PTEs and PMDs. It requires adjustment to conditions when we can reuse
> the page on write-protection fault.
> 
> For PTE fault we can't reuse the page if it's part of huge page.
> 
> For PMD we can only reuse the page if nobody else maps the huge page or
> it's part. We can do it by checking page_mapcount() on each sub-page,
> but it's expensive.
> 
> The cheaper way is to check page_count() to be equal 1: every mapcount
> takes page reference, so this way we can guarantee, that the PMD is the
> only mapping.
> 
> This approach can give false negative if somebody pinned the page, but
> that doesn't affect correctness.
>
Then we have to try more to allocate THP if pinned?
Are we adding new cost?

Hillf

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