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From:   John Hubbard <jhubbard@nvidia.com>
To:     Andrew Morton <akpm@linux-foundation.org>
CC:     Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
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        John Hubbard <jhubbard@nvidia.com>
Subject: [PATCH v5 10/12] mm/gup: /proc/vmstat: pin_user_pages (FOLL_PIN) reporting
Date:   Thu, 6 Feb 2020 19:37:33 -0800
Message-ID: <20200207033735.308000-11-jhubbard@nvidia.com>
In-Reply-To: <20200207033735.308000-1-jhubbard@nvidia.com>
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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 | 33 +++++++++++++++++++----
 include/linux/mmzone.h                    |  2 ++
 mm/gup.c                                  | 14 ++++++++++
 mm/vmstat.c                               |  2 ++
 4 files changed, 46 insertions(+), 5 deletions(-)

diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core=
-api/pin_user_pages.rst
index 24641f1a1eba..2e939ff10b86 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 rel=
eased
 since the system was booted, via two new /proc/vmstat entries: ::
=20
-    /proc/vmstat/nr_foll_pin_requested
-    /proc/vmstat/nr_foll_pin_requested
+    /proc/vmstat/nr_foll_pin_acquired
+    /proc/vmstat/nr_foll_pin_released
=20
-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 a=
ny
+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 hu=
ge
+  pages: the head page is refcounted once for each tail or head page in th=
e 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 pa=
ge.
+  Becaused of the pin count behavior described above in "nr_foll_pin_acqui=
red",
+  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 =3D=3D nr_foll_pin_acquired
+
+(...unless it was already out of balance due to a long-term RDMA pin being=
 in
+place.)
=20
 Other diagnostics
 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
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
 };
=20
diff --git a/mm/gup.c b/mm/gup.c
index 4d0d94405639..ae503c51bc7f 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 =3D 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_he=
ad(struct page *page,
 		if (hpage_pincount_available(page))
 			hpage_pincount_add(page, refs);
=20
+		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED,
+				    orig_refs);
+
 		return page;
 	}
=20
@@ -158,6 +163,8 @@ bool __must_check try_grab_page(struct page *page, unsi=
gned 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);
 	}
=20
 	return true;
@@ -178,6 +185,7 @@ static bool __unpin_devmap_managed_user_page(struct pag=
e *page)
=20
 	count =3D page_ref_sub_return(page, refs);
=20
+	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)
=20
 	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);
=20
@@ -2258,6 +2268,8 @@ static int record_subpages(struct page *page, unsigne=
d long addr,
=20
 static void put_compound_head(struct page *page, int refs, unsigned int fl=
ags)
 {
+	int orig_refs =3D refs;
+
 	if (flags & FOLL_PIN) {
 		if (hpage_pincount_available(page))
 			hpage_pincount_sub(page, refs);
@@ -2273,6 +2285,8 @@ static void put_compound_head(struct page *page, int =
refs, unsigned int flags)
 	if (refs > 1)
 		page_ref_sub(page, refs - 1);
 	put_page(page);
+
+	mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_RELEASED, orig_refs);
 }
=20
 #ifdef CONFIG_ARCH_HAS_HUGEPD
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[] =3D {
 	"nr_dirtied",
 	"nr_written",
 	"nr_kernel_misc_reclaimable",
+	"nr_foll_pin_acquired",
+	"nr_foll_pin_released",
=20
 	/* enum writeback_stat_item counters */
 	"nr_dirty_threshold",
--=20
2.25.0