Re: [PATCH v4 10/12] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting
From: John Hubbard <jhubbard@nvidia.com>
Date: 2020-02-05 23:58:59
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On 2/5/20 3:43 AM, Kirill A. Shutemov wrote:
On Tue, Feb 04, 2020 at 03:41:15PM -0800, John Hubbard wrote:quoted
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: updated 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>Please, clarify semantics for huge page. An user may want to know if we count huge page as one pin-acquired or by number of pages.
OK, I've added this for the next version:
diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
index 5776ad1ed5e4..2e939ff10b86 100644
--- a/Documentation/core-api/pin_user_pages.rst
+++ b/Documentation/core-api/pin_user_pages.rst@@ -211,6 +211,33 @@ since the system was booted, via two new /proc/vmstat entries: :: /proc/vmstat/nr_foll_pin_acquired /proc/vmstat/nr_foll_pin_released +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.) + Other diagnostics =================
thanks, -- John Hubbard NVIDIA
Otherwise looks good (given Jan concern is addressed).