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
