__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>

1 files changed, 10 insertions, 14 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 9920db455f05..353f82e09e63 100644
--- 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;