[PATCH 5/6] fuse: rework fuse_perform_write()

[Maxim Patlasov <mpatlasov@parallels.com>]

The patch allocates as many page pointers in fuse_req as needed to cover
interval [pos .. pos+len-1]. FUSE_WR_PAGES macro is introduced to hide this
cumbersome arithmetics.
---
 fs/fuse/file.c |   15 +++++++++++----
 1 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 9a6dcc6..84cc83c 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -815,7 +815,8 @@ static size_t fuse_send_write_pages(struct fuse_req *req, struct file *file,
 
 static ssize_t fuse_fill_write_pages(struct fuse_req *req,
 			       struct address_space *mapping,
-			       struct iov_iter *ii, loff_t pos)
+			       struct iov_iter *ii, loff_t pos,
+			       unsigned nr_pages)
 {
 	struct fuse_conn *fc = get_fuse_conn(mapping->host);
 	unsigned offset = pos & (PAGE_CACHE_SIZE - 1);
@@ -875,11 +876,16 @@ static ssize_t fuse_fill_write_pages(struct fuse_req *req,
 		if (!fc->big_writes)
 			break;
 	} while (iov_iter_count(ii) && count < fc->max_write &&
-		 req->num_pages < FUSE_MAX_PAGES_PER_REQ && offset == 0);
+		 req->num_pages < nr_pages && offset == 0);
 
 	return count > 0 ? count : err;
 }
 
+#define FUSE_WR_PAGES(pos, len) min_t(unsigned,				      \
+				      ((pos + len - 1) >> PAGE_CACHE_SHIFT) - \
+				      (pos >> PAGE_CACHE_SHIFT) + 1,	      \
+				      FUSE_MAX_PAGES_PER_REQ)
+
 static ssize_t fuse_perform_write(struct file *file,
 				  struct address_space *mapping,
 				  struct iov_iter *ii, loff_t pos)
@@ -895,14 +901,15 @@ static ssize_t fuse_perform_write(struct file *file,
 	do {
 		struct fuse_req *req;
 		ssize_t count;
+		unsigned nr_pages = FUSE_WR_PAGES(pos, iov_iter_count(ii));
 
-		req = fuse_get_req_multipage(fc, FUSE_MAX_PAGES_PER_REQ);
+		req = fuse_get_req_multipage(fc, nr_pages);
 		if (IS_ERR(req)) {
 			err = PTR_ERR(req);
 			break;
 		}
 
-		count = fuse_fill_write_pages(req, mapping, ii, pos);
+		count = fuse_fill_write_pages(req, mapping, ii, pos, nr_pages);
 		if (count <= 0) {
 			err = count;
 		} else {