[PATCH 2/5] mm/gup: introduce pin_user_page()

[John Hubbard]

pin_user_page() is the FOLL_PIN equivalent of get_page().

This was always a missing piece of the pin/unpin API calls (early
reviewers of pin_user_pages() asked about it, in fact), but until now,
it just wasn't needed. Finally though, now that the Direct IO pieces in
block/bio are about to be converted to use FOLL_PIN, it turns out that
there are some cases in which get_page() and get_user_pages_fast() were
both used. Converting those sites requires a drop-in replacement for
get_page(), which this patch supplies.

[1] and [2] provide some background about pin_user_pages() in general.

[1] "Explicit pinning of user-space pages":
    https://lwn.net/Articles/807108/

[2] Documentation/core-api/pin_user_pages.rst

Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
---
 include/linux/mm.h |  2 ++
 mm/gup.c           | 30 ++++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1983e08f5906..bee26614f430 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1149,6 +1149,8 @@ static inline void get_page(struct page *page)
 	page_ref_inc(page);
 }
 
+void pin_user_page(struct page *page);
+
 bool __must_check try_grab_page(struct page *page, unsigned int flags);
 
 static inline __must_check bool try_get_page(struct page *page)
diff --git a/mm/gup.c b/mm/gup.c
index ae096ea7583f..2cae5bbbc862 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -123,6 +123,36 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
 	return NULL;
 }
 
+/*
+ * pin_user_page() - elevate the page refcount, and mark as FOLL_PIN
+ *
+ * This the FOLL_PIN equivalent of get_page(). It is intended for use when the
+ * page will be released via unpin_user_page().
+ */
+void pin_user_page(struct page *page)
+{
+	int refs = 1;
+
+	page = compound_head(page);
+
+	VM_BUG_ON_PAGE(page_ref_count(page) <= 0, page);
+
+	if (hpage_pincount_available(page))
+		hpage_pincount_add(page, 1);
+	else
+		refs = GUP_PIN_COUNTING_BIAS;
+
+	/*
+	 * Similar to try_grab_compound_head(): even if using the
+	 * hpage_pincount_add/_sub() routines, be sure to
+	 * *also* increment the normal page refcount field at least
+	 * 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);
+}
+
 /**
  * try_grab_page() - elevate a page's refcount by a flag-dependent amount
  *
-- 
2.28.0