[PATCH 3/4] add rwmem type backed by pages

[Zach Brown <zach.brown@oracle.com>]

This lets callers specify a region of memory to read from or write to with
an array of page/offset/len tuples.  There have been specific requests to
do this from servers which want to do O_DIRECT from the kernel.  (knfsd?)

This could also be used by places which currently hold a kmap() and call
fop->write.  ecryptfs_write_lower_page_segment() is one such caller.
---
 fs/rwmem.c            |   66 +++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/rwmem.h |   16 ++++++++++++
 2 files changed, 82 insertions(+), 0 deletions(-)

diff --git a/fs/rwmem.c b/fs/rwmem.c
index 0433ba4..c87e8a4 100644
--- a/fs/rwmem.c
+++ b/fs/rwmem.c
@@ -90,3 +90,69 @@ struct rwmem_ops rwmem_iovec_ops = {
 	.seg_bytes		= rwmem_iovec_seg_bytes,
 	.get_seg_pages		= rwmem_iovec_get_seg_pages,
 };
+
+void rwmem_pages_init(struct rwmem *rwm)
+{
+	struct rwmem_pages *rwp = container_of(rwm, struct rwmem_pages, rwmem);
+	struct pgol *pgol;
+	unsigned long i;
+
+	rwm->total_bytes = 0;
+	rwm->nr_pages = rwm->nr_segs;
+	rwm->boundary_bits = 0;
+
+	for (i = 0; i < rwm->nr_segs; i++) {
+		pgol = &rwp->pgol[i];
+
+		rwm->total_bytes += pgol->len;
+		rwm->boundary_bits |= pgol->offset | pgol->len;
+	}
+}
+
+/*
+ * Returns the offset of the start of a segment within its first page.
+ */
+unsigned long rwmem_pages_seg_page_offset(struct rwmem *rwm, unsigned long i)
+{
+	struct rwmem_pages *rwp = container_of(rwm, struct rwmem_pages, rwmem);
+	BUG_ON(i >= rwm->nr_segs);
+	return rwp->pgol[i].offset;
+}
+
+/*
+ * Returns the total bytes in the given segment.
+ */
+unsigned long rwmem_pages_seg_bytes(struct rwmem *rwm, unsigned long i)
+{
+	struct rwmem_pages *rwp = container_of(rwm, struct rwmem_pages, rwmem);
+	BUG_ON(i >= rwm->nr_segs);
+	return rwp->pgol[i].len;
+}
+
+/*
+ * For now each page is its own seg.
+ */
+int rwmem_pages_get_seg_pages(struct rwmem *rwm, unsigned long i,
+			      unsigned long *cursor, struct page **pages,
+			      unsigned long max_pages, int write)
+{
+	struct rwmem_pages *rwp = container_of(rwm, struct rwmem_pages, rwmem);
+	int ret = 0;
+
+	BUG_ON(i >= rwm->nr_segs);
+	BUG_ON(*cursor != 0);
+
+	if (max_pages) {
+		pages[0] = rwp->pgol[i].page;
+		get_page(pages[0]);
+		ret = 1;
+	}
+	return ret;
+}
+
+struct rwmem_ops rwmem_pages_ops = {
+	.init			= rwmem_pages_init,
+	.seg_page_offset	= rwmem_pages_seg_page_offset,
+	.seg_bytes		= rwmem_pages_seg_bytes,
+	.get_seg_pages		= rwmem_pages_get_seg_pages,
+};
diff --git a/include/linux/rwmem.h b/include/linux/rwmem.h
index 666f9f4..47019f0 100644
--- a/include/linux/rwmem.h
+++ b/include/linux/rwmem.h
@@ -26,4 +26,20 @@ struct rwmem_iovec {
 };
 struct rwmem_ops rwmem_iovec_ops;
 
+/*
+ * How many times do we need this in subsystems before we make a universal
+ * struct?  (bio_vec, skb_frag_struct, pipe_buffer)
+ */
+struct pgol {
+	struct page *page;
+	unsigned int offset;
+	unsigned int len;
+};
+
+struct rwmem_pages {
+	struct rwmem	rwmem;
+	struct pgol	*pgol;
+};
+struct rwmem_ops rwmem_pages_ops;
+
 #endif
-- 
1.5.2.2