[PATCH v3 2/6] mm, gup: Ensure real head page is ref-counted when using hugepages

[Kirill A. Shutemov]

On Mon, May 22, 2017 at 02:36:00PM +0100, Punit Agrawal wrote:
> When speculatively taking references to a hugepage using
> page_cache_add_speculative() in gup_huge_pmd(), it is assumed that the
> page returned by pmd_page() is the head page. Although normally true,
> this assumption doesn't hold when the hugepage comprises of successive
> page table entries such as when using contiguous bit on arm64 at PTE or
> PMD levels.
> 
> This can be addressed by ensuring that the page passed to
> page_cache_add_speculative() is the real head or by de-referencing the
> head page within the function.
> 
> We take the first approach to keep the usage pattern aligned with
> page_cache_get_speculative() where users already pass the appropriate
> page, i.e., the de-referenced head.
> 
> Apply the same logic to fix gup_huge_[pud|pgd]() as well.

Hm. Okay. But I'm kinda surprise that this is the only place that need to
be adjusted.

Have you validated all other pmd_page() use-cases?

-- 
 Kirill A. Shutemov