[PATCH v4 10/12] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting

John Hubbard
2020-02-04T23:42:02Z

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>
---
Documentation/core-api/pin_user_pages.rst | 8 ++------
include/linux/mmzone.h | 2 ++
mm/gup.c | 10 ++++++++++
mm/vmstat.c | 2 ++
4 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
index dd21ea140ef4..194204d74a50 100644
--- a/Documentation/core-api/pin_user_pages.rst
+++ b/Documentation/core-api/pin_user_pages.rst
@@ -208,12 +208,8 @@ 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
-
-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.
+ /proc/vmstat/nr_foll_pin_acquired
+ /proc/vmstat/nr_foll_pin_released

Other diagnostics
=================
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 462f6873905a..4bca42eeb439 100644
--- a/include/linux/mmzone.h
+++ b/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
};

diff --git a/mm/gup.c b/mm/gup.c
index 4d0d94405639..7c543849181b 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -86,6 +86,8 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
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_grab_compound_head(struct page *page,
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 page *page, unsigned int flags)
* 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_page(struct page *page)

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);

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 78d53378db99..c9c0d71f917f 100644
--- a/mm/vmstat.c
+++ b/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",
--
2.25.0