[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>
diff --git a/mm/filemap.c b/mm/filemap.c
index 4082b3b..648f2c0 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2125,6 +2125,12 @@
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 @@
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 536979f..3f2a343 100644
--- a/mm/filemap.h
+++ b/mm/filemap.h
@@ -88,7 +88,7 @@
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 @@
iov++;
base = 0;
}
- }
+ } while (bytes);
*iovp = iov;
*basep = base;
}