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author | Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com> | 2010-10-28 01:30:07 (GMT) |
---|---|---|
committer | Theodore Ts'o <tytso@mit.edu> | 2010-10-28 01:30:07 (GMT) |
commit | 0c9169ccad4aed233fdd49e95da4eada2536a06d (patch) | |
tree | 2c89c49f8aa48dd781747e4f9d801ad06e5517b9 | |
parent | e0d10bfa91b0a089a9e2c378b5c42f4e96171d95 (diff) | |
download | linux-0c9169ccad4aed233fdd49e95da4eada2536a06d.tar.xz |
ext4: fix potential infinite loop in ext4_da_writepages()
On linux-2.6.36-rc2, if we execute the following script, we can hang
the system when the /bin/sync command is executed:
========================================================================
#!/bin/sh
echo -n "HANG UP TEST: "
/bin/dd if=/dev/zero of=/tmp/img bs=1k count=1 seek=1M 2> /dev/null
/sbin/mkfs.ext4 -Fq /tmp/img
/bin/mount -o loop -t ext4 /tmp/img /mnt
/bin/dd if=/dev/zero of=/mnt/file bs=1 count=1 \
seek=$((16*1024*1024*1024*1024-4096)) 2> /dev/null
/bin/sync
/bin/umount /mnt
echo "DONE"
exit 0
========================================================================
We can see the following backtrace if we get the kdump when this
hangup occurs:
======================================================================
kthread()
=> bdi_writeback_thread()
=> wb_do_writeback()
=> wb_writeback()
=> writeback_inodes_wb()
=> writeback_sb_inodes()
=> writeback_single_inode()
=> ext4_da_writepages() ---+
^ infinite |
| loop |
+-------------+
======================================================================
The reason why this hangup happens is described as follows:
1) We write the last extent block of the file whose size is the filesystem
maximum size.
2) "BH_Delay" flag is set on the buffer_head of its block.
3) - the member, "m_lblk" of struct mpage_da_data is 4294967295 (UINT_MAX)
- the member, "m_len" of struct mpage_da_data is 1
mpage_put_bnr_to_bhs() which is called via ext4_da_writepages()
cannot clear "BH_Delay" flag of the buffer_head because the type of
m_lblk is ext4_lblk_t and then m_lblk + m_len is overflow.
Therefore an infinite loop occurs because ext4_da_writepages()
cannot write the page (which corresponds to the block) since
"BH_Delay" flag isn't cleared.
----------------------------------------------------------------------
static void mpage_put_bnr_to_bhs(struct mpage_da_data *mpd,
struct ext4_map_blocks *map)
{
...
int blocks = map->m_len;
...
do {
// cur_logical = 4294967295
// map->m_lblk = 4294967295
// blocks = 1
// *** map->m_lblk + blocks == 0 (OVERFLOW!) ***
// (cur_logical >= map->m_lblk + blocks) => true
if (cur_logical >= map->m_lblk + blocks)
break;
----------------------------------------------------------------------
NOTE: Mounting with the nodelalloc option will avoid this codepath,
and thus, avoid this hang
Signed-off-by: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
-rw-r--r-- | fs/ext4/inode.c | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index 50f3bba..1e824a3 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -2105,7 +2105,7 @@ static void mpage_put_bnr_to_bhs(struct mpage_da_data *mpd, } while ((bh = bh->b_this_page) != head); do { - if (cur_logical >= map->m_lblk + blocks) + if (cur_logical > map->m_lblk + (blocks - 1)) break; if (buffer_delay(bh) || buffer_unwritten(bh)) { |