From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: Bug introduced in 3b93f911d5
Date: Fri, 8 Aug 2014 16:54:07 +0100
Message-ID: <20140808155407.GT18016@ZenIV.linux.org.uk>
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Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>
To: Anton Altaparmakov <aia21@cam.ac.uk>
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On Fri, Aug 08, 2014 at 12:11:39AM +0100, Anton Altaparmakov wrote:
> Hi Al,
> 
> Was just looking at __generic_file_write_iter() and found a bug in the code that you added in 3b93f911d5.
> 
> Consider the case where generic_file_direct_write() returns a partial write, i.e. written > 0 && written < count.
> 
> Also consider that the following generic_perform_write() fails with an error, i.e. status < 0.

*nod*

What we ought to do, AFAICS, is this:

diff --git a/mm/filemap.c b/mm/filemap.c
index 900edfa..8163e04 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2584,7 +2584,7 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		 * that this differs from normal direct-io semantics, which
 		 * will return -EFOO even if some bytes were written.
 		 */
-		if (unlikely(status < 0) && !written) {
+		if (unlikely(status < 0)) {
 			err = status;
 			goto out;
 		}

Note that we return written ? written : err, so assignment to err will be
the right thing both when status < 0 && written == 0 and when status < 0 &&
written > 0.  In the latter case err will be simply ignored.

Objections?