__generic_file_write_iter: keep ->ki_pos and return value consistent

A side effect worth noting: in O_APPEND case we set ->ki_pos early,
so if it turns out to be an error or a zero-length write, we'll
end up with ->ki_pos modified.  Safe, since all callers never
look at the ->ki_pos after the call of __generic_file_write_iter()
returning non-positive, all the way to caller of ->write_iter() and
those discard ->ki_pos when getting that.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/mm/filemap.c b/mm/filemap.c
index 9920db4..353f82e 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2557,7 +2557,6 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
        struct file *file = iocb->ki_filp;
        struct address_space * mapping = file->f_mapping;
        struct inode    *inode = mapping->host;
-       loff_t          pos = iocb->ki_pos;
        ssize_t         written = 0;
        ssize_t         err;
        ssize_t         status;
@@ -2565,7 +2564,7 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 
        /* We can write back this queue in page reclaim */
        current->backing_dev_info = inode_to_bdi(inode);
-       err = generic_write_checks(file, &pos, &count, S_ISBLK(inode->i_mode));
+       err = generic_write_checks(file, &iocb->ki_pos, &count, S_ISBLK(inode->i_mode));
        if (err)
                goto out;
 
@@ -2583,9 +2582,9 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
                goto out;
 
        if (io_is_direct(file)) {
-               loff_t endbyte;
+               loff_t pos, endbyte;
 
-               written = generic_file_direct_write(iocb, from, pos);
+               written = generic_file_direct_write(iocb, from, iocb->ki_pos);
                /*
                 * If the write stopped short of completing, fall back to
                 * buffered writes.  Some filesystems do this for writes to
@@ -2593,13 +2592,10 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
                 * not succeed (even if it did, DAX does not handle dirty
                 * page-cache pages correctly).
                 */
-               if (written < 0 || written == count || IS_DAX(inode))
+               if (written < 0 || !iov_iter_count(from) || IS_DAX(inode))
                        goto out;
 
-               pos += written;
-               count -= written;
-
-               status = generic_perform_write(file, from, pos);
+               status = generic_perform_write(file, from, pos = iocb->ki_pos);
                /*
                 * If generic_perform_write() returned a synchronous error
                 * then we want to return the number of bytes which were
@@ -2611,15 +2607,15 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
                        err = status;
                        goto out;
                }
-               iocb->ki_pos = pos + status;
                /*
                 * We need to ensure that the page cache pages are written to
                 * disk and invalidated to preserve the expected O_DIRECT
                 * semantics.
                 */
                endbyte = pos + status - 1;
-               err = filemap_write_and_wait_range(file->f_mapping, pos, endbyte);
+               err = filemap_write_and_wait_range(mapping, pos, endbyte);
                if (err == 0) {
+                       iocb->ki_pos = endbyte + 1;
                        written += status;
                        invalidate_mapping_pages(mapping,
                                                 pos >> PAGE_CACHE_SHIFT,
@@ -2631,9 +2627,9 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
                         */
                }
        } else {
-               written = generic_perform_write(file, from, pos);
-               if (likely(written >= 0))
-                       iocb->ki_pos = pos + written;
+               written = generic_perform_write(file, from, iocb->ki_pos);
+               if (likely(written > 0))
+                       iocb->ki_pos += written;
        }
 out:
        current->backing_dev_info = NULL;