[PATCH] generic_file_buffered_write(): handle zero-length iovec segments

The recent generic_file_write() deadlock fix caused
generic_file_buffered_write() to loop inifinitely when presented with a
zero-length iovec segment.  Fix.

Note that this fix deliberately avoids calling ->prepare_write(),
->commit_write() etc with a zero-length write.  This is because I don't trust
all filesystems to get that right.

This is a cautious approach, for 2.6.17.x.  For 2.6.18 we should just go ahead
and call ->prepare_write() and ->commit_write() with the zero length and fix
any broken filesystems.  So I'll make that change once this code is stabilised
and backported into 2.6.17.x.

The reason for preferring to call ->prepare_write() and ->commit_write() with
the zero-length segment: a zero-length segment _should_ be sufficiently
uncommon that this is the correct way of handling it.  We don't want to
optimise for poorly-written userspace at the expense of well-written
userspace.

Cc: "Vladimir V. Saveliev" <vs@namesys.com>
Cc: Neil Brown <neilb@suse.de>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: Greg KH <greg@kroah.com>
Cc: <stable@kernel.org>
Cc: walt <wa1ter@myrealbox.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

mm/filemap.c

@@ -2125,6 +2125,12 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 			break;
 		}
 
+		if (unlikely(bytes == 0)) {
+			status = 0;
+			copied = 0;
+			goto zero_length_segment;
+		}
+
 		status = a_ops->prepare_write(file, page, offset, offset+bytes);
 		if (unlikely(status)) {
 			loff_t isize = i_size_read(inode);
@@ -2154,7 +2160,8 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
 			page_cache_release(page);
 			continue;
 		}
-		if (likely(copied > 0)) {
+zero_length_segment:
+		if (likely(copied >= 0)) {
 			if (!status)
 				status = copied;
 

mm/filemap.h

@@ -88,7 +88,7 @@ filemap_set_next_iovec(const struct iovec **iovp, size_t *basep, size_t bytes)
 	const struct iovec *iov = *iovp;
 	size_t base = *basep;
 
-	while (bytes) {
+	do {
 		int copy = min(bytes, iov->iov_len - base);
 
 		bytes -= copy;
@@ -97,7 +97,7 @@ filemap_set_next_iovec(const struct iovec **iovp, size_t *basep, size_t bytes)
 			iov++;
 			base = 0;
 		}
-	}
+	} while (bytes);
 	*iovp = iov;
 	*basep = base;
 }