diff options
Diffstat (limited to 'Documentation/core-api')
| -rw-r--r-- | Documentation/core-api/pin_user_pages.rst | 33 |
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 ========== |