pin_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 the overall effort to convert
things to pin_user_page*() and unpin_user_page*().
[1] "Explicit pinning of user-space pages":
https://lwn.net/Articles/807108/
[2] Documentation/core-api/pin_user_pages.rst
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
---
include/linux/mm.h | 2 ++
mm/gup.c | 33 +++++++++++++++++++++++++++++++++
2 files changed, 35 insertions(+)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ca6e6a81576b..24240cf66c44 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1154,6 +1154,8 @@ static inline void get_page(struct page *page)
page_ref_inc(page);
}
+void pin_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..a3a4bfae224a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -123,6 +123,39 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
return NULL;
}
+/*
+ * pin_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().
+ *
+ * Unlike pin_user_page*(), pin_page() may be used on nearly any page, not just
+ * userspace-allocated pages.
+ */
+void pin_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
