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authorJohn Hubbard <jhubbard@nvidia.com>2020-04-01 21:05:29 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2020-04-02 09:35:27 -0700
commit3faa52c03f440d1b9ddef18c4f189f4790d52d7e (patch)
treebb6a2fef66352f853961d6a90c379f01b0a9ada4 /Documentation/core-api
parent94202f126f698691f8865906ad6a68203e5dde8c (diff)
downloadlinux-3faa52c03f440d1b9ddef18c4f189f4790d52d7e.tar.bz2
mm/gup: track FOLL_PIN pages
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 [jhubbard@nvidia.com: add kerneldoc] Link: http://lkml.kernel.org/r/20200307021157.235726-1-jhubbard@nvidia.com [imbrenda@linux.ibm.com: if pin fails, we need to unpin, a simple put_page will not be enough] Link: http://lkml.kernel.org/r/20200306132537.783769-2-imbrenda@linux.ibm.com [akpm@linux-foundation.org: fix put_compound_head defined but not used] Suggested-by: Jan Kara <jack@suse.cz> Suggested-by: Jérôme Glisse <jglisse@redhat.com> Signed-off-by: John Hubbard <jhubbard@nvidia.com> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Reviewed-by: Jan Kara <jack@suse.cz> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Ira Weiny <ira.weiny@intel.com> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Christoph Hellwig <hch@infradead.org> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Dave Chinner <david@fromorbit.com> Cc: Jason Gunthorpe <jgg@ziepe.ca> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Michal Hocko <mhocko@suse.com> Cc: Mike Kravetz <mike.kravetz@oracle.com> Cc: Shuah Khan <shuah@kernel.org> Cc: Vlastimil Babka <vbabka@suse.cz> Link: http://lkml.kernel.org/r/20200211001536.1027652-7-jhubbard@nvidia.com Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/core-api')
-rw-r--r--Documentation/core-api/pin_user_pages.rst6
1 files changed, 3 insertions, 3 deletions
diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
index 1d490155ecd7..9829345428f8 100644
--- a/Documentation/core-api/pin_user_pages.rst
+++ b/Documentation/core-api/pin_user_pages.rst
@@ -173,8 +173,8 @@ CASE 4: Pinning for struct page manipulation only
-------------------------------------------------
Here, normal GUP calls are sufficient, so neither flag needs to be set.
-page_dma_pinned(): the whole point of pinning
-=============================================
+page_maybe_dma_pinned(): the whole point of pinning
+===================================================
The whole point of marking pages as "DMA-pinned" or "gup-pinned" is to be able
to query, "is this page DMA-pinned?" That allows code such as page_mkclean()
@@ -186,7 +186,7 @@ and debates (see the References at the end of this document). It's a TODO item
here: fill in the details once that's worked out. Meanwhile, it's safe to say
that having this available: ::
- static inline bool page_dma_pinned(struct page *page)
+ static inline bool page_maybe_dma_pinned(struct page *page)
...is a prerequisite to solving the long-running gup+DMA problem.