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authorJohn Hubbard <jhubbard@nvidia.com>2020-04-01 21:05:37 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2020-04-02 09:35:27 -0700
commit1970dc6f5226416957ad0cc70ab47386ed3195a6 (patch)
treeaf17c6c4e04a023603d0f568ced8d6a7a705827c /Documentation/core-api
parent47e29d32afba11b13efb51f03154a8cf22fb4360 (diff)
mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting
Now that pages are "DMA-pinned" via pin_user_page*(), and unpinned via unpin_user_pages*(), we need some visibility into whether all of this is working correctly. Add two new fields to /proc/vmstat: nr_foll_pin_acquired nr_foll_pin_released These are documented in Documentation/core-api/pin_user_pages.rst. They represent the number of pages (since boot time) that have been pinned ("nr_foll_pin_acquired") and unpinned ("nr_foll_pin_released"), via pin_user_pages*() and unpin_user_pages*(). In the absence of long-running DMA or RDMA operations that hold pages pinned, the above two fields will normally be equal to each other. Also: update Documentation/core-api/pin_user_pages.rst, to remove an earlier (now confirmed untrue) claim about a performance problem with /proc/vmstat. Also: update Documentation/core-api/pin_user_pages.rst to rename the new /proc/vmstat entries, to the names listed here. Signed-off-by: John Hubbard <jhubbard@nvidia.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: Jérôme Glisse <jglisse@redhat.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/20200211001536.1027652-9-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.rst33
1 files changed, 28 insertions, 5 deletions
diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
index 7e5dd8b1b3f2..5c8a5f89756b 100644
--- a/Documentation/core-api/pin_user_pages.rst
+++ b/Documentation/core-api/pin_user_pages.rst
@@ -208,12 +208,35 @@ has the following new calls to exercise the new pin*() wrapper functions:
You can monitor how many total dma-pinned pages have been acquired and released
since the system was booted, via two new /proc/vmstat entries: ::
- /proc/vmstat/nr_foll_pin_requested
- /proc/vmstat/nr_foll_pin_requested
+ /proc/vmstat/nr_foll_pin_acquired
+ /proc/vmstat/nr_foll_pin_released
-Those are both going to show zero, unless CONFIG_DEBUG_VM is set. This is
-because there is a noticeable performance drop in unpin_user_page(), when they
-are activated.
+Under normal conditions, these two values will be equal unless there are any
+long-term [R]DMA pins in place, or during pin/unpin transitions.
+
+* nr_foll_pin_acquired: This is the number of logical pins that have been
+ acquired since the system was powered on. For huge pages, the head page is
+ pinned once for each page (head page and each tail page) within the huge page.
+ This follows the same sort of behavior that get_user_pages() uses for huge
+ pages: the head page is refcounted once for each tail or head page in the huge
+ page, when get_user_pages() is applied to a huge page.
+
+* nr_foll_pin_released: The number of logical pins that have been released since
+ the system was powered on. Note that pages are released (unpinned) on a
+ PAGE_SIZE granularity, even if the original pin was applied to a huge page.
+ Becaused of the pin count behavior described above in "nr_foll_pin_acquired",
+ the accounting balances out, so that after doing this::
+
+ pin_user_pages(huge_page);
+ for (each page in huge_page)
+ unpin_user_page(page);
+
+...the following is expected::
+
+ nr_foll_pin_released == nr_foll_pin_acquired
+
+(...unless it was already out of balance due to a long-term RDMA pin being in
+place.)
References
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