Re: [PATCH v4 07/12] mm/gup: track FOLL_PIN pages

[Kirill A. Shutemov]

On Tue, Feb 04, 2020 at 03:41:12PM -0800, John Hubbard wrote:
> Add tracking of pages that were pinned via FOLL_PIN. This tracking is
> implemented via overloading of page->_refcount: pins are added by
> adding GUP_PIN_COUNTING_BIAS (1024) to the refcount. This provides a
> fuzzy indication of pinning, and it can have false positives (and that's
> OK). Please see the pre-existing
> Documentation/core-api/pin_user_pages.rst for details.
> 
> As mentioned in pin_user_pages.rst, callers who effectively set FOLL_PIN
> (typically via pin_user_pages*()) are required to ultimately free such
> pages via unpin_user_page().
> 
> Please also note the limitation, discussed in pin_user_pages.rst under
> the "TODO: for 1GB and larger huge pages" section. (That limitation will
> be removed in a following patch.)
> 
> The effect of a FOLL_PIN flag is similar to that of FOLL_GET, and may be
> thought of as "FOLL_GET for DIO and/or RDMA use".
> 
> Pages that have been pinned via FOLL_PIN are identifiable via a
> new function call:
> 
>    bool page_maybe_dma_pinned(struct page *page);
> 
> What to do in response to encountering such a page, is left to later
> patchsets. There is discussion about this in [1], [2], [3], and [4].
> 
> This also changes a BUG_ON(), to a WARN_ON(), in follow_page_mask().
> 
> [1] Some slow progress on get_user_pages() (Apr 2, 2019):
>     https://lwn.net/Articles/784574/
> [2] DMA and get_user_pages() (LPC: Dec 12, 2018):
>     https://lwn.net/Articles/774411/
> [3] The trouble with get_user_pages() (Apr 30, 2018):
>     https://lwn.net/Articles/753027/
> [4] LWN kernel index: get_user_pages():
>     https://lwn.net/Kernel/Index/#Memory_management-get_user_pages
> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Suggested-by: Jan Kara <jack@xxxxxxx>
> Suggested-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>

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

-- 
 Kirill A. Shutemov