[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: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 10 insertions, 3 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 4082b3b3cea7..648f2c0c8e18 100644
--- a/mm/filemap.c
+++ b/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;
 
diff --git a/mm/filemap.h b/mm/filemap.h
index 536979fb4ba7..3f2a343c6015 100644
--- a/mm/filemap.h
+++ b/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;
 }