From: John Hubbard <jhubbard@xxxxxxxxxx>
Subject: 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.
Link: http://lkml.kernel.org/r/20200211001536.1027652-9-jhubbard@xxxxxxxxxx
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/core-api/pin_user_pages.rst | 33 ++++++++++++++++----
include/linux/mmzone.h | 2 +
mm/gup.c | 13 +++++++
mm/vmstat.c | 2 +
4 files changed, 45 insertions(+), 5 deletions(-)
--- a/Documentation/core-api/pin_user_pages.rst~mm-gup-proc-vmstat-pin_user_pages-foll_pin-reporting
+++ a/Documentation/core-api/pin_user_pages.rst
@@ -208,12 +208,35 @@ has the following new calls to exercise
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
==========
--- a/include/linux/mmzone.h~mm-gup-proc-vmstat-pin_user_pages-foll_pin-reporting
+++ a/include/linux/mmzone.h
@@ -243,6 +243,8 @@ enum node_stat_item {
NR_DIRTIED, /* page dirtyings since bootup */
NR_WRITTEN, /* page writings since bootup */
NR_KERNEL_MISC_RECLAIMABLE, /* reclaimable non-slab kernel pages */
+ NR_FOLL_PIN_ACQUIRED, /* via: pin_user_page(), gup flag: FOLL_PIN */
+ NR_FOLL_PIN_RELEASED, /* pages returned via unpin_user_page() */
NR_VM_NODE_STAT_ITEMS
};
--- a/mm/gup.c~mm-gup-proc-vmstat-pin_user_pages-foll_pin-reporting
+++ a/mm/gup.c
@@ -86,6 +86,8 @@ static __maybe_unused struct page *try_g
if (flags & FOLL_GET)
return try_get_compound_head(page, refs);
else if (flags & FOLL_PIN) {
+ int orig_refs = refs;
+
/*
* When pinning a compound page of order > 1 (which is what
* hpage_pincount_available() checks for), use an exact count to
@@ -104,6 +106,9 @@ static __maybe_unused struct page *try_g
if (hpage_pincount_available(page))
hpage_pincount_add(page, refs);
+ mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED,
+ orig_refs);
+
return page;
}
@@ -158,6 +163,8 @@ bool __must_check try_grab_page(struct p
* once, so that the page really is pinned.
*/
page_ref_add(page, refs);
+
+ mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED, 1);
}
return true;
@@ -178,6 +185,7 @@ static bool __unpin_devmap_managed_user_
count = page_ref_sub_return(page, refs);
+ mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, 1);
/*
* devmap page refcounts are 1-based, rather than 0-based: if
* refcount is 1, then the page is free and the refcount is
@@ -228,6 +236,8 @@ void unpin_user_page(struct page *page)
if (page_ref_sub_and_test(page, refs))
__put_page(page);
+
+ mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, 1);
}
EXPORT_SYMBOL(unpin_user_page);
@@ -2014,6 +2024,9 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
static void put_compound_head(struct page *page, int refs, unsigned int flags)
{
if (flags & FOLL_PIN) {
+ mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED,
+ refs);
+
if (hpage_pincount_available(page))
hpage_pincount_sub(page, refs);
else
--- a/mm/vmstat.c~mm-gup-proc-vmstat-pin_user_pages-foll_pin-reporting
+++ a/mm/vmstat.c
@@ -1168,6 +1168,8 @@ const char * const vmstat_text[] = {
"nr_dirtied",
"nr_written",
"nr_kernel_misc_reclaimable",
+ "nr_foll_pin_acquired",
+ "nr_foll_pin_released",
/* enum writeback_stat_item counters */
"nr_dirty_threshold",
_
