If there is a error while copying data from userspace into the page
cache during a write(2) system call, in data=journal mode, in
ext4_journalled_write_end() were using page_zero_new_buffers() from
fs/buffer.c. Unfortunately, this sets the buffer dirty flag, which is
no good if journalling is enabled. This is a long-standing bug that
goes back for years and years in ext3, but a combination of (a)
data=journal not being very common, (b) in many case it only results
in a warning message. and (c) only very rarely causes the kernel hang,
means that we only really noticed this as a problem when commit
998ef75ddb caused this failure to happen frequently enough to cause
generic/208 to fail when run in data=journal mode.
The fix is to have our own version of this function that doesn't call
mark_dirty_buffer(), since we will end up calling
ext4_handle_dirty_metadata() on the buffer head(s) in questions very
shortly afterwards in ext4_journalled_write_end().
Thanks to Dave Hansen and Linus Torvalds for helping to identify the
root cause of the problem.
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
---
fs/ext4/inode.c | 34 +++++++++++++++++++++++++++++++++-
1 file changed, 33 insertions(+), 1 deletion(-)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index ae52e32..0a589bb 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1181,6 +1181,38 @@ errout:
return ret ? ret : copied;
}
+/*
+ * This is a private version of page_zero_new_buffers() which doesn't
+ * set the buffer to be dirty, since in data=journalled mode we need
+ * to call ext4_handle_dirty_metadata() instad.
+ */
+static void zero_new_buffers(struct page *page, unsigned from, unsigned to)
+{
+ unsigned int block_start = 0, block_end;
+ struct buffer_head *head, *bh;
+
+ bh = head = page_buffers(page);
+ do {
+ block_end = block_start + bh->b_size;
+ if (buffer_new(bh)) {
+ if (block_end > from && block_start < to) {
+ if (!PageUptodate(page)) {
+ unsigned start, size;
+
+ start = max(from, block_start);
+ size = min(to, block_end) - start;
+
+ zero_user(page, start, size);
+ set_buffer_uptodate(bh);
+ }
+ clear_buffer_new(bh);
+ }
+ }
+ block_start = block_end;
+ bh = bh->b_this_page;
+ } while (bh != head);
+}
+
static int ext4_journalled_write_end(struct file *file,
struct address_space *mapping,
loff_t pos, unsigned len, unsigned copied,
@@ -1207,7 +1239,7 @@ static int ext4_journalled_write_end(struct file *file,
if (copied < len) {
if (!PageUptodate(page))
copied = 0;
- page_zero_new_buffers(page, from+copied, to);
+ zero_new_buffers(page, from+copied, to);
}
ret = ext4_walk_page_buffers(handle, page_buffers(page), from,
--
2.5.0
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