[PATCH v2 6/8] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting

John Hubbard
2020-01-29T03:24:45Z

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_requested
nr_foll_pin_returned

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_requested") and unpinned ("nr_foll_pin_returned"),
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.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
include/linux/mmzone.h | 2 ++
mm/gup.c | 21 +++++++++++++++++++++
mm/vmstat.c | 2 ++
3 files changed, 25 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 462f6873905a..392868bc4763 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_REQUESTED, /* via: pin_user_page(), gup flag: FOLL_PIN */
+ NR_FOLL_PIN_RETURNED, /* pages returned via unpin_user_page() */
NR_VM_NODE_STAT_ITEMS
};

diff --git a/mm/gup.c b/mm/gup.c
index 03e7a5cfa6a9..d536bda383c4 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -29,6 +29,19 @@ struct follow_page_context {
unsigned int page_mask;
};

+#ifdef CONFIG_DEBUG_VM
+static inline void __update_proc_vmstat(struct page *page,
+ enum node_stat_item item, int count)
+{
+ mod_node_page_state(page_pgdat(page), item, count);
+}
+#else
+static inline void __update_proc_vmstat(struct page *page,
+ enum node_stat_item item, int count)
+{
+}
+#endif
+
static void hpage_pincount_add(struct page *page, int refs)
{
VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
@@ -86,6 +99,8 @@ 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 +119,7 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
if (hpage_pincount_available(page))
hpage_pincount_add(page, refs);

+ __update_proc_vmstat(page, NR_FOLL_PIN_REQUESTED, orig_refs);
return page;
}

@@ -159,6 +175,8 @@ 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);
+
+ __update_proc_vmstat(page, NR_FOLL_PIN_REQUESTED, 1);
}

return true;
@@ -179,6 +197,7 @@ static bool __unpin_devmap_managed_user_page(struct page *page)

count = page_ref_sub_return(page, refs);

+ __update_proc_vmstat(page, NR_FOLL_PIN_RETURNED, 1);
/*
* devmap page refcounts are 1-based, rather than 0-based: if
* refcount is 1, then the page is free and the refcount is
@@ -229,6 +248,8 @@ void unpin_user_page(struct page *page)

if (page_ref_sub_and_test(page, refs))
__put_page(page);
+
+ __update_proc_vmstat(page, NR_FOLL_PIN_RETURNED, 1);
}
EXPORT_SYMBOL(unpin_user_page);

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 78d53378db99..b56808bae1b4 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_requested",
+ "nr_foll_pin_returned",

/* enum writeback_stat_item counters */
"nr_dirty_threshold",
--
2.25.0