Re: [PATCH v3 08/12] mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages

[Kirill A. Shutemov]

On Fri, Jan 31, 2020 at 07:40:25PM -0800, John Hubbard wrote:
> For huge pages (and in fact, any compound page), the
> GUP_PIN_COUNTING_BIAS scheme tends to overflow too easily, each tail
> page increments the head page->_refcount by GUP_PIN_COUNTING_BIAS
> (1024). That limits the number of huge pages that can be pinned.
> 
> This patch removes that limitation, by using an exact form of pin
> counting for compound pages of order > 1. The "order > 1" is required
> because this approach uses the 3rd struct page in the compound page, and
> order 1 compound pages only have two pages, so that won't work there.

Could you update the comment for HPAGE_PMD_ORDER < 2 check in
hugepage_init() to reflect addtional user for the condition.
> 
> A new struct page field, hpage_pinned_refcount, has been added,
> replacing a padding field in the union (so no new space is used).
> 
> This enhancement also has a useful side effect: huge pages and compound
> pages (of order > 1) do not suffer from the "potential false positives"
> problem that is discussed in the page_dma_pinned() comment block. That
> is because these compound pages have extra space for tracking things, so
> they get exact pin counts instead of overloading page->_refcount.
> 
> Documentation/core-api/pin_user_pages.rst is updated accordingly.
> 
> Suggested-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov