md: prepare for managing resync I/O pages in clean way

Now resync I/O use bio's bec table to manage pages,
this way is very hacky, and may not work any more
once multipage bvec is introduced.

So introduce helpers and new data structure for
managing resync I/O pages more cleanly.
Signed-off-by: NMing Lei <tom.leiming@gmail.com>
Signed-off-by: NShaohua Li <shli@fb.com>

drivers/md/md.h

@@ -723,4 +723,54 @@ static inline void mddev_check_writesame(struct mddev *mddev, struct bio *bio)
 #define RESYNC_BLOCK_SIZE (64*1024)
 #define RESYNC_PAGES ((RESYNC_BLOCK_SIZE + PAGE_SIZE-1) / PAGE_SIZE)
 
+/* for managing resync I/O pages */
+struct resync_pages {
+	unsigned	idx;	/* for get/put page from the pool */
+	void		*raid_bio;
+	struct page	*pages[RESYNC_PAGES];
+};
+
+static inline int resync_alloc_pages(struct resync_pages *rp,
+				     gfp_t gfp_flags)
+{
+	int i;
+
+	for (i = 0; i < RESYNC_PAGES; i++) {
+		rp->pages[i] = alloc_page(gfp_flags);
+		if (!rp->pages[i])
+			goto out_free;
+	}
+
+	return 0;
+
+out_free:
+	while (--i >= 0)
+		put_page(rp->pages[i]);
+	return -ENOMEM;
+}
+
+static inline void resync_free_pages(struct resync_pages *rp)
+{
+	int i;
+
+	for (i = 0; i < RESYNC_PAGES; i++)
+		put_page(rp->pages[i]);
+}
+
+static inline void resync_get_all_pages(struct resync_pages *rp)
+{
+	int i;
+
+	for (i = 0; i < RESYNC_PAGES; i++)
+		get_page(rp->pages[i]);
+}
+
+static inline struct page *resync_fetch_page(struct resync_pages *rp,
+					     unsigned idx)
+{
+	if (WARN_ON_ONCE(idx >= RESYNC_PAGES))
+		return NULL;
+	return rp->pages[idx];
+}
+
 #endif /* _MD_MD_H */